KHOWLEDGE ZONE

He was a king, a commander, and a conqueror. Alexander the Great was so powerful some people called him a god. He was one of the greatest generals in history, and he built a vast empire that extended from the Mediterranean Sea to India.

ALEXANDER’S EARLY LIFE

Alexander was born in Macedonia in 356 bc. His father, King Philip II of Macedonia, hired the famous Greek philosopher Aristotle to tutor young Alexander. In the summer of 336 bc, Philip was murdered by one of his bodyguards. Alexander then became king.

Many people in Macedonia plotted against the young king, but Alexander was shrewd. He quickly ordered the execution of all the conspirators. At the same time, some Greek cities ruled by Macedonia rebelled and others threatened to seek independence. Alexander crushed the rebellions and restored Macedonian rule.

INVASION OF ASIA

Next, in 334 bc, Alexander turned his attention toward the Persian Empire (now Iran) in southwest Asia. Alexander led Macedonian and Greek soldiers to attack Darius III, Persia’s king. Their armies met at Issus in Syria in 333 bc, and fought a fierce battle. Alexander won, and Darius fled.

CONTROL OF EGYPT

Alexander then led his soldiers south, into Egypt. Alexander seized power from the pharaoh, who ruled Egypt on behalf of the Persians. The grateful Egyptians saw Alexander as a person who freed them, and they crowned him pharaoh. At the mouth of the Nile River in northern Egypt, Alexander founded a new city. He named the city Alexandria, and it became a famous center of learning.

CONQUEROR OF PERSIA

In 331 bc, Alexander led his troops back north into Persia. King Darius was eager for revenge. Alexander and Darius fought another great battle, this time at Gaugamela. Once again, Alexander won. The battle at Gaugamela ended centuries of Persian rule in Asia.

Alexander then turned south and conquered other important Persian cities. At Persepolis, he burned down Darius’s palace to show he had conquered the Persian Empire. In 330 bc, Alexander went north to find Darius again. This time, Darius was killed by his own men as he fled.

WORLD EMPIRE

Alexander was a military genius and a great explorer. But he also had a grand ambition. He wanted to rule a world empire where people could live in peace with one another. From 330 to 327 bc, Alexander led his soldiers east, through Afghanistan and into Central Asia. As he travelled, he built more cities. He recruited soldiers, merchants, and scholars from many lands to settle there.

In 326 bc, Alexander turned south, into India. But by then his men were tired and weak. They were far from home in an unknown land. The soldiers rebelled and refused to go farther. Reluctantly, Alexander turned back. By 323 bc, he reached Babylon in Iraq. While there he caught a fever and died at the age of 33. His empire was divided among his generals.

Watch a bolt of lightning flash across the sky. Flip a switch and light up your bedroom. Click the remote and see the TV come on. What do all of these things have in common? Electricity.

WHAT IS ELECTRICITY?

Electricity is a powerful force of nature. Electricity is everywhere in the universe. Electrical forces hold water, metals, and all other kinds of matter together. You can walk and run because electric signals go through your nerves from your brain to your muscles. The signals tell your muscles where to move.

Electricity makes many machines work. Electricity makes bulbs light up and runs motors in saws, fans, hairdryers, and other appliances. The computer you are using works because of electricity.

WHERE DOES ELECTRICITY COME FROM?

Electricity starts with atoms. Atoms are tiny bits of matter much too small for you to see. Everything in the universe is made up of atoms.

Atoms have two main parts: a center or nucleus, and electrons that orbit or go around the nucleus. Electricity comes from electrons. You cannot see electrons and you cannot see electricity. You can see what electricity does because of electric charge and electric energy.

WHAT IS ELECTRIC CHARGE?

Electric charge comes from the parts inside atoms. There are two kinds of electric charge called positive charge and negative charge. Positive charge comes from the nucleus of an atom. Negative charge comes from electrons. Atoms do not normally have any overall charge because their positive and negative charges cancel each other out. Charge comes when electrons move away from an atom.

Positive charge is just the opposite of negative charge. Positive and negative charges pull toward each other. The pull of positive and negative charges makes two kinds of electricity—static electricity and electric current.

WHAT IS STATIC ELECTRICITY?

Did you ever get a shock after walking across a carpet and touching a metal doorknob? That shock came from static electricity. Huge amounts of static electricity cause lightning.

Electrons that move away from their atoms cause static electricity. You can make static electricity by rubbing certain materials together. Run a plastic comb through your hair. Be sure your hair is clean and dry. Electrons jump from your hair to the comb. This gives the comb a negative electric charge. Your hair loses electrons. This gives your hair a positive electric charge. Hold the comb above your head and watch some of your hairs stand on end. Your hair stands on end because the positive and negative charges are pulling toward one another.

Static electricity also causes lightning. The pull of positive and negative charges between clouds and the ground creates a huge spark. The spark is actually the charges moving very quickly toward each other. Lightning can also be caused by opposite charges inside one cloud, between two clouds, or between clouds and the air.

WHAT MAKES LAMPS LIGHT UP?
Electric current makes lamps and all other electric devices work. Electric current is actually electrons moving in a big loop.

Something must give the electrons a push to get them moving. Batteries can start electrons flowing. Batteries are a source of electric energy. A battery, two wires, and a light bulb can make an electric circuit. The current starts flowing from the battery through a wire to the light bulb. The other wire carries the electric current back to the battery. If you cut the wire, the electric current stops. Switches on an electric circuit turn the current on and off. This is how a wall switch works to turn lights on and off in your home.

The electric energy in your home does not come from batteries. You plug appliances into electric outlets in your walls. The electric energy in the outlets comes from electric power plants.

HOW DO POWER PLANTS WORK?

Huge electric power plants generate or make electricity. Steam or falling water in dams make big machines called turbines turn. The turbine drives another machine called an electric generator. The generator makes electricity.

Long power lines carry electricity from power plants to your home. Wires inside your home bring the electric energy to light bulbs, TVs, microwaves, and your computer.

WHO DISCOVERED ELECTRICITY?

For thousands of years people knew that a material called amber mysteriously pulled on some materials. The ancient Greeks called amber elektron. Scientists in Europe in the 1600s and early 1700s called the materials that amber attracted electrics.

Benjamin Franklin, an American printer, patriot, and inventor, experimented with electricity. He thought lightning and electricity were the same thing. He did a dangerous experiment in the mid-1700s to find out. Franklin flew a kite during a thunderstorm. He attached a metal key to the kite string. An electric charge ran down the wet kite string to the key. The charge made a spark when it hit the key. This showed Franklin that lightning was electricity. He was lucky he was not killed.

Many other scientists have experimented with electricity since Benjamin Franklin. They learned how to make electricity with batteries. They found that electricity would go through wires. An American inventor named Thomas Alva Edison invented many things that use electricity, including the electric light bulb.

People ran and hid when they saw Viking ships coming. “From the fury of the Northmen, good Lord deliver us!” they prayed. The Vikings were daring, skillful sailors and frightening warriors. For 300 years, from ad 800 to 1100, they terrorized much of northern Europe.

HARSH HOMELAND

The Vikings came from Scandinavia, an area in northern Europe that now includes the countries of Norway, Denmark, and Sweden. The climate there is harsh, with bitter winters and short summers. Viking families struggled to survive. They grew oats and barley during the short growing season. They also hunted deer, bear, and wolves in the woods and caught fish, seals, and whales from the ocean.

RAIDERS AND SETTLERS

Viking warriors sailed in fast, sleek drakkar (dragon ships). They had a single sail but the warriors also rowed them with oars.

The Vikings sailed south to attack villages and churches around the coasts of Europe. They seized valuables and kidnapped men, women, and children to sell as slaves. The Vikings demanded danegeld (money for protection) from towns and threatened to attack again if they weren’t paid. Boatloads of Vikings landed in Scotland, Ireland, England, and France. They killed local rulers and set up their own kingdoms. Vikings also took control of parts of Russia and the Ukraine.

Brave Viking adventurers headed west across the Atlantic to settle in Iceland and Greenland. About ad 1000, they became the first Europeans to land in North America. They called the new land Vinland and built a settlement there, but the settlement did not survive.

SKILLED AND CIVILIZED

Wherever they traveled, the Vikings brought violence. But they also brought their own rich culture. They were expert craftspeople, carving wood, stone, bone, and antlers into useful everyday objects. They forged sharp iron axes, swords, and spears. They made the best ships in Europe, and the best jewelry. They were also shrewd traders. They traveled long distances to sell amber, furs, and slaves in such cities as Constantinople (now Istanbul, Turkey) and Baghdad (in Iraq).

Viking people were proud of their highly developed society. They held community meetings, called things, to punish criminals and make laws. They set up carved memorial stones to honor respected leaders. Viking poets memorized stories and histories of brave deeds.

Viking traditions survive throughout northern Europe, and Viking tales of gods, heroes, and monsters still inspire stirring works of art.

They can cut metal. They can cut glass. They are billions of years old and form deep within Earth. Some are so rare and beautiful that people will pay a fortune to get one! They are diamonds.

WHAT ARE DIAMONDS?

Diamonds are made of carbon, a chemical element. That’s the same stuff that’s in the tip of a pencil. But isn’t carbon soft, not hard?

Pencil carbon is so soft that it rubs off on paper. But diamond carbon is so hard it can cut almost anything. In fact, diamonds are the hardest things in nature. What makes the difference? The atoms inside a diamond have a special crystal shape. This makes them very hard.

Some diamonds can sparkle brilliantly. Such diamonds are rare. They are precious and valuable, and people wear them as jewelry. But most diamonds are small and contain flaws. These are worth much less.

HOW DO DIAMONDS FORM?

Big diamonds form deep under the ground. The heat and pressure at great depths melts carbon-rich rocks. Diamonds form out of this molten (melted) rock. The diamonds are later brought to Earth’s surface by currents of molten rock. The entire process can take billions of years!

Smaller diamonds can form closer to Earth’s surface. Movements of the planet’s crust lift these stones up from the depths.

WHAT CAN DIAMONDS DO?

You can scratch or cut just about anything with a diamond—except another diamond. On a scale of hardness from 1 to 10 with 10 as the hardest, diamonds are a 10.

Because they are so hard, diamonds are great for drills, saws, and other cutting tools. That makes them very useful in mining and manufacturing.

SYNTHETIC AND IMITATION DIAMONDS

Are there any shortcuts to making diamonds? Actually, there are. People have learned how to make artificial diamonds and other gems. Diamonds made by people are called synthetic diamonds.

Synthetic diamonds look and act like the real thing. They’re as hard as natural ones. In fact, most diamonds used for industrial purposes are synthetic.

Imitation diamonds aren’t diamonds at all. They’re usually made from inexpensive materials that sparkle brightly, such as quartz or glass. But they aren’t as hard as natural or synthetic diamonds. Imitation diamonds are used mostly in jewelry.

WHY ARE DIAMONDS CUT?

Diamonds found in the ground are rough. They don’t shine like the gems in jewelry stores. They look greasy or dirty. So how do you make diamonds really sparkle? You cut them.

Cutting a diamond gives the stone edges that bring out the shine. Diamond cutters trim away flaws such as cracks or cloudy spots.

It takes careful planning to cut a diamond. The goal is to make the diamond as large and as valuable as possible. Even so, a cutter slices away about half of a diamond’s original size. After polishing, the cut diamond is ready to sell.

WHAT MAKES DIAMONDS WORTH SO MUCH?

Diamonds are judged by the “4 Cs.” These are color, clarity, carat weight, and cut.

Color: When diamonds form, small amounts of minerals get inside them. This changes their color. Colorless stones are very desirable. Yellowish or brownish ones are not.

Clarity: Can you see flaws in the diamond? A diamond with great clarity has no visible flaws. The clearer a stone is, the greater its value.

Carat weight: The heavier a diamond is, the better. Because large, heavy diamonds are less common, they are more valuable. A diamond that weighs 2 carats or more is usually considered large.

Cut: Diamond cutters decide on the shape and proportion of a stone. An attractive cut makes a diamond more desirable.

HOW BIG CAN DIAMONDS GET?

The largest natural diamond ever found is the famous Cullinan diamond. It was discovered in South Africa in 1905. The Cullinan tipped the scales at an amazing 3,106 carats!

What do you do with a diamond that big? The Cullinan was cut into 105 different gems. The largest of these is called the Star of Africa. It is the largest cut diamond in the world. Today, the Star of Africa sits on the end of the British royal scepter.

Take a really deep breath. Feel how your chest gets bigger and bigger. Your chest gets bigger because your lungs are filling up with air. You cannot see air, but air is all around you. You can feel it when the wind blows.

Earth’s atmosphere is made of air. An atmosphere is made up of the gases that surround a planet.

WHAT IS AIR?

Air is a mixture of several different gases. The main gases in air are nitrogen, oxygen, and argon. Air also contains smaller amounts of hydrogen, carbon dioxide, water vapor, helium, and other gases. Oxygen is the most important gas for animals. Animals must breathe oxygen in order to live.

Carbon dioxide is the most important gas for plants. Plants use carbon dioxide and sunlight to make food. Plants give off oxygen. Animals turn the oxygen back into carbon dioxide when they breathe.

TAKING AIR WITH YOU

You can go to places where there is no air. There is no air underwater, but you can dive underwater. You can stay underwater a short time just by holding your breath. Air tanks let you stay underwater for a long time. Scuba divers wear tanks on their backs. The tanks are filled with gases that make up air. The divers breathe the gases through hoses.

There is less and less air the higher up you go. People gasp for breath at the tops of tall mountains. Airplanes must carry air. Once the airplane gets up high, air is pumped into the cabin where passengers sit. Astronauts have to take all the air they need with them—there’s no air in space!

They called him “the man with a million stories.” People flocked to Marco Polo’s home to hear him tell exciting tales about his travels in distant lands.

Marco Polo won fame for his journeys across Asia. He wrote a book about his travels that became one of the most famous travel guides in history.

EARLY LIFE

Marco Polo was born in 1254 to a family of merchants. His home was Venice, Italy. Venetian merchants bought and sold valuable Chinese goods, including precious silk cloth. Such goods were brought to Europe along an ancient route known as the Silk Road. The merchants also used the route to travel east on trading missions.

Marco’s mother died when he was a young boy. His family taught him to be a merchant. He learned how to read, write, calculate, and use foreign money.

In 1269, Marco’s father and uncle, Niccolò and Maffeo, returned to Venice after visiting China. In China, they had met the Mongol conqueror Kublai Khan. The khan invited the Polos to return. He asked them to bring Christian scholars to explain the Christian religion to him.

TRIP TO CHINA

In 1271, Niccolò and Maffeo set out for China again. Marco, then 17 years old, joined his father and uncle for the trip. Two priests also traveled with the Polos. But the route was dangerous, and the priests soon turned back.

It took the Polos four difficult years to reach China. The journey led across deserts and high mountains. They passed through wild countryside where bandits lurked, ready to rob and kill. They braved heat and cold, floods, deep snowdrifts, and blinding sandstorms. At last they reached the summer palace of Kublai Khan at Shangdu.

The khan welcomed the Polos warmly. He offered Marco a job. Marco accepted, and the Polos lived in China for the next 17 years. Marco traveled on many special missions across the khan’s kingdom and to distant lands. When Marco returned from his missions, he told the khan vivid stories about the people and lands he visited.

Over time, the Polos worried that Kublai Khan would not allow them to leave. Several times they had asked the khan for permission to return to Europe. But the khan enjoyed his visitors so much that he would not grant their wish. Finally, the khan changed his mind.

RETURN TO VENICE

In 1292, Kublai Khan asked Marco to escort a Mongol princess to Persia. The Polos traveled by ship from China to the Persian Gulf. Then the Polos headed for Venice, finally reaching home in 1295. Marco had been away so long that nobody recognized him!

We know about Marco’s travels because, in 1298, he became a prisoner of war. He shared his cell with a writer named Rustichello, who helped Marco turn his stories into a book. Rustichello added some details of his own. But much of Marco’s book seems to be true!

Marco Polo returned to Venice after his release from prison. He died in 1324. But his book remained popular for centuries. Merchants, mapmakers, and explorers all looked to the book for information about Asian lands. Even the navigator Christopher Columbus owned a copy!

Did you ever look at the Moon and think you could see a face? Sometimes dark spots on the Moon look like eyes, a nose, and a mouth. People used to talk about “the man in the Moon.” They would joke about the Moon being made of cheese with holes in it.

The Moon is the second brightest thing in our sky, after the Sun. The Moon doesn’t make its own light. Light rays from the Sun bounce off it and make it shine. The Moon is closer to Earth than any other body in our solar system.

WHAT’S ON THE MOON?


In the 1600s, the famous Italian scientist Galileo was the first person to look at the Moon through a telescope. He saw dark spots that he thought were oceans. He called them maria, the Latin word for “seas.” Galileo thought the light areas were large landmasses called continents.

Today, we know a lot more about the Moon. We know that nothing lives on the Moon, and there are no oceans. The maria are dry, flat plains covered with rocks. The Moon is the only place in space that human beings have visited.

TOUCHING THE MOON

The first astronauts landed on the Moon in 1969. They traveled in a United States spacecraft named Apollo 11. The astronauts set up experiments on the Moon and brought some moon rocks back to Earth. Later, five more Apollo missions explored different parts of the Moon. The astronauts on these missions brought back more rocks and soil.

Scientists learned many things about the Moon from the Apollo space missions. They also learned from other spacecraft that orbited (went around) the Moon. Some of these spacecraft sent robot landers down to the surface of the Moon.

SPACE ROCKS AND CRATERS

The dry, gray Moon might seem like a boring place now. But you should have seen it several billion years ago.

Many times over the past two or three billion years, chunks of rock and ice have come whizzing toward the Moon. The space rocks and ice are asteroids and comets. They slam into the Moon’s surface. The biggest ones came just after Earth and the other planets were formed. When they hit the Moon, these large objects threw up tons of rock and dust. There are billions of big and small pits on the Moon made by the space rocks. These pits are called craters.

ANCIENT VOLCANOES

If you went to the Moon, you’d see the dark-colored maria. Scientists think the dark gray rock is lava (melted rock). They believe that billions of years ago, red-hot rock gushed up from volcanoes on the Moon. The lava flowed over the Moon’s surface. It filled in low places, including some of the big craters. Then the lava cooled to make the Moon’s gray rocks.

The lava also left round hills on the Moon called domes and carved grooves called rilles.

ROUGH HIGHLANDS

There are rough and mountainous places all over the Moon. Scientists call these places highlands.

There are highlands on the far side of the Moon but almost no maria. Only one side of the Moon faces Earth, so you can never see the far side of the Moon. Scientists learned what the far side looks like from pictures taken by orbiting spacecraft.

HOT DAYS AND COLD NIGHTS

The astronauts who walked on the Moon had to wear big space suits. The space suits provided air for the astronauts to breathe, because there is no air on the Moon. The suits also kept the astronauts cool during hot Moon days and warm during cold Moon nights.

With no atmosphere to protect it, Moon temperatures can be very high and very low. It can be 261° Fahrenheit (127° Celsius) at noon during a Moon day—hotter than boiling water! It can be as cold as -279° Fahrenheit (-173° Celsius) on a Moon night. Days and nights on the Moon each last about two weeks.

Days and nights are long because the Moon turns very slowly. It takes the Moon about 27 days to make one turn. Earth turns once every 24 hours.

ICE ON THE MOON?

There is no water on the Moon, but scientists think that there may be ice. Two spacecraft in the 1990s saw signs of the ice. If there is ice on the Moon, it could help future explorers stay there longer.

The signs of ice were found in deep craters at the north and south poles of the Moon. Because these craters are always in shadow, it stays very cold there—about -364° Fahrenheit (-220° Celsius).

THE MOON FROM EARTH

The Moon always seems to change shape. Sometimes it looks like a round ball in the sky. Sometimes it is a thin sliver. But the Moon does not really change shape. What happens to it?

The Moon reflects light from the Sun. How you see the reflected sunlight depends on where the Moon is. The Moon orbits (goes around) Earth. Sometimes it is between the Sun and Earth, and you can’t see any reflected sunlight. This is called the new moon.

Sometimes Earth is between the Moon and the Sun. You can see all of the reflected sunlight. The Moon looks round. This is called a full moon.

The rest of the time, you see only part of the reflected sunlight from the Moon. The reflected sunlight looks like slivers of Moon. It takes about 27 days to go from a new moon to a full moon and back to a new moon again.

WHERE THE MOON CAME FROM

No one knows for sure how the Moon was formed. By testing moon rocks, scientists have learned that the Moon is about 4.6 billion years old. This is the same age as the solar system.

Scientists think that at that time something as big as a planet crashed into Earth. The collision blasted huge pieces of Earth into space. Some of the pieces came together to make the Moon.

Scientists continue to study moon rocks for clues. There is still much to learn about the Moon.

Hi all Viewers,

I am Mr. Kith Mean, 31 years old, a lecturer of Business & Management at privat university in Cambodia and a Marketing Research Executive for a private company.

The purpose of making this blog is to share the universal knowledge to the world. In fact, this scientific knowledge was composed by the scientists, and I just only compile it for the easy reading.

Hopefully, you will enjoy reading the scientific articles and watching the scientific movies because they are about the natural phenominon, and you always keep following them for the broadly knowledgable purpose.

Anyway, I will take my best effort to update the information in the blog for you to make you more enjoyable, informative, and knowledgable.

Regards

Captain James Cook was one of the world’s greatest explorers. He sailed around the world twice. He was the first European to reach Hawaii and New Zealand, and he sailed farther south than any European had ever gone.

People marvelled over the places, people, and things Cook described. Before Cook, nobody in Europe knew about penguins or kangaroos!

EARLY LIFE

Cook was born in 1728 on a farm in northern England. At the age of 18, he went to work for a shipping company. In 1755, Cook joined the British Royal Navy. His ship was sent to Canada, to make maps of land that Britain had conquered from France.

FIRST VOYAGE TO THE PACIFIC

In 1768, Cook sailed to the South Pacific Ocean, with artists and scientists. Officially, their task was to observe the planet Venus. But Britain also hoped that Cook would find a mysterious “Southern Continent” that some sailors claimed to have seen. Cook wanted to take control of it for the British king.

Cook reached New Zealand in 1770. No other European had been there. He sailed around New Zealand and then explored eastern Australia.

A SCIENTIFIC EXPLORER

Cook drew many detailed maps and kept careful records of all he had seen on his voyage. He described native peoples of the South Pacific and their cultures. His artists sketched wildlife, and his scientists collected unusual plants and animals to take back.

Cook’s careful work caused a sensation when he arrived home in Britain in 1771. No other expedition had gathered so much information, so thoroughly and scientifically.

Cook also won fame for keeping his sailors healthy. He wondered if a lack of fresh fruits and vegetables caused scurvy, a fatal disease common among sailors on long voyages. He stocked his ship with sauerkraut (pickled cabbage) and forced his crew to eat it. During stops at port, he ordered his crew to eat fresh foods. As a result, few of his sailors became seriously ill.

SECOND VOYAGE TO THE PACIFIC

From 1772 to 1775, Cook made a second voyage to the South Pacific Ocean. This time, he sailed farther south than anyone before him. He saw penguins and icebergs. He sailed all the way around Antarctica. But he found no land where people might live.

FINAL VOYAGE

In 1776, Cook set off on a third voyage. This time, Cook wanted to look for the Northwest Passage. This was a possible sea route north of Canada linking Europe and Asia. Before sailing north, he explored several islands in the Pacific. He landed in Hawaii in 1778, becoming the first European to do so.

From Hawaii, Cook sailed to North America. He was the first European to set foot on Vancouver Island off the coast of British Columbia. Throughout 1778 he explored the northwest coast of North America, but he failed to find the Northwest Passage. In 1779, Cook returned to Hawaii, where he was killed in a quarrel with natives over a stolen boat.

People in the 1800s didn’t know what to think about a new invention called the automobile. No one was sure it would catch on. In those days, people often traveled in carriages pulled by horses. So when the first automobiles appeared, people nicknamed them “horseless carriages.”

The first automobiles looked a lot like horse carriages. That was the style people knew. But the automobile soon took on a look that was all its own. The modern automobile has a hood and fenders. It has a roof, sides, and four wheels. It has seats where the driver and passengers sit. Modern automobiles are commonly called cars or autos.

Few machines are as important as cars. You can ride to school in one. Adults can drive one to work. You can drive in a car to shopping malls. You can take long vacations traveling in an automobile.

TYPES OF CARS

The typical passenger car can carry up to six people. Larger vehicles called minivans are like big cars. They can usually carry up to eight people. Pickups or trucks are built to carry cargo.

Sport-utility vehicles (SUVs) are made for driving in all types of conditions, including mud or snow. Sports cars are built for power and good road handling. Many sports cars have room for just two passengers.

Racing cars are specially designed to compete on tracks and courses. Most racing cars are built to be lightweight and very fast. Because they are made for racing, they usually are not suited for driving on public streets.

WHAT MAKES CARS GO?

A car gets power from its engine. Most auto engines burn gasoline. Gasoline goes through fuel lines from a gas tank to the engine.

When it burns fuel, the engine makes exhaust gases. These gases go out through pipes called the exhaust system.

Moving parts hooked up to the engine are called the drivetrain. The drivetrain carries mechanical energy from the engine to the wheels. The turning wheels make an automobile go.

Springs and shock absorbers give passengers a smoother ride on bumpy roads. Electrical parts make the headlights, turn signals, horn, radio, and windshield wipers work. The electrical parts also help start the car. Brake parts rub against the wheels to slow the car down. Seat belts and air bags help protect you in an accident.

HOW DO YOU DRIVE A CAR?

People don’t just jump into cars and start driving. First, they must get their learner’s permit. Local auto bureaus can tell you how. Driver-education classes teach people how to drive a car. Students learn how a car works and the rules of safe driving.

To start a car, you sit in the driver’s seat. Turning a key in the ignition starts the engine. Moving the car’s gearshift connects the engine to the drivetrain. Pressing the gas pedal on the floor sends fuel to the engine. The harder you press, the faster the car goes.

To make the car turn left or right, you turn the steering wheel in the direction you want to go. To make the car move forward or backward, you use the gearshift.

What about stopping? Press your foot down on the brake pedal. The brakes will press against the wheels, making them slow down and then stop turning.

WHEN WERE THE FIRST CARS MADE?

The first cars were built in the 1700s. They were powered by steam engines. In England, steam-powered cars weren’t allowed on the roads. They were run like trains on private railroad tracks!

Auto racing became popular in the late 1800s and early 1900s. Some early racing cars had steam engines. These included the American-made Stanley Steamer. In 1906, a Stanley Steamer hit a speed of more than 121 miles per hour (195 kilometers per hour), setting a new land speed record.

Other cars made at the time ran on electricity from batteries. People liked them because they were quiet and less likely to scare horses and people. Still other cars had gasoline engines. The first gasoline-powered cars were loud, slow, and unreliable. But over time, the cars were improved, and more people wanted to drive them.

HENRY FORD INVENTS THE ASSEMBLY LINE

In the United States, a businessman named Henry Ford started the Ford Motor Company in 1903. His company made two famous kinds of cars: the Model A and the Model T.

Ford invented the factory assembly line for making cars. Workers in one place along the assembly line worked on just one part of the car. Other workers, in another area of the assembly line, worked on another part of the car.

Automobiles made this way were not very expensive. Ordinary people could afford them. The Model T became one of the biggest-selling automobiles of all time. Henry Ford sold more than 15 million Model T cars before his company stopped making them in 1927.

MODERN CARS

Modern cars are much better than earlier models. They are easier to drive and have advanced safety features such as air bags. Engines are more efficient and powerful. Cars are quieter and more comfortable inside.

Today, cars are more popular than ever. They are the main form of transportation for many people in the United States and Canada. Many people own more than one car.

WHAT ABOUT AIR POLLUTION?

The exhaust gases that come from burning gasoline can pollute the air. These gases contain chemicals that cause a smoky pollution called smog. The worst smog forms in cities. Exhaust gases also contain a gas called carbon dioxide. Scientists think carbon dioxide pollution is making Earth’s climate warmer.

Scientists and engineers are working to reduce pollution from cars. They have made cars that burn less gasoline. They have designed exhaust systems that give off less pollution. They have also developed efficient hybrid cars.

HYBRID CARS

Many people believe hybrid cars could be a big help in reducing pollution. Hybrid cars are automobiles that run partly on gasoline and partly on some other fuel. Most hybrid cars use electricity from batteries. Scientists are also experimenting with hybrids that run on energy from sunlight and other sources.

First you see a blinding flash of light, brighter than the Sun. Moments later, a huge ball of fire appears, brilliant orange. The fireball begins to rise into the sky. Soon it widens at the top and is shaped like a mushroom. A thundering sound and blast of heat reach you 15 miles (24 kilometers) away. You are seeing the explosion of the world’s first nuclear weapon, on July 16, 1945, in a New Mexico desert.


WHY DO WE HAVE NUCLEAR WEAPONS?
Nuclear weapons are the most destructive weapons ever made. Building a nuclear weapon was a top-secret project during World War II. Scientists had been working on this weapon—the atomic bomb—for three years by 1945. Almost nobody else, except the president of the United States, knew about this work. The secret effort to build a nuclear weapon was called the Manhattan Project. By 1942, when the Manhattan Project began, Germany had conquered much of Europe and was out to conquer the rest. The United States had just joined the war. The United States and its allies were afraid that Germany would develop an atomic bomb first. Then Germany would win the war. The United States and its allies had to beat Germany to the bomb. Germany had already surrendered by the time the atomic bomb was ready. But Japan was still fighting the war. To end the war quickly, the United States dropped two atomic bombs on Japan. The bombs killed at least 100,000 people and destroyed the cities of Hiroshima and Nagasaki. Japan surrendered soon afterward. The nuclear age had begun.

WHY WAS THERE A NUCLEAR ARMS RACE?
The nuclear arms race was a buildup of nuclear weapons after World War II. When the war ended, scientists knew that it was possible to build nuclear bombs far more powerful and destructive than the first atomic bomb. Some people, including scientists, thought it was wrong to build these weapons of mass destruction. Others feared that the Soviet Union would make them first. By the late 1940s, the Cold War pitted the United States and its allies against the Soviet Union and its allies. Each side feared an attack from the other side, though their armies did not actually fight during the Cold War. Everyone knew that a war using nuclear weapons would be a terrible disaster. A nuclear war would kill millions of people and possibly end life on Earth. Each side believed that having a large supply of nuclear weapons would frighten the other side and stop it from starting a nuclear war. If one side attacked, the other side would strike back with even more nuclear bombs. And so began a race to have more nuclear weapons than the other side. Luckily, no nuclear attacks happened after World War II.

HOW DO NUCLEAR WEAPONS WORK?
A nuclear weapon gets its name and its explosive power from the nucleus (core) of an atom. Atoms are tiny building blocks of matter much too small to see. An atomic bomb works by fissioning (splitting) the nuclei of atoms of the metals uranium or plutonium. It is sometimes called a fission weapon. A hydrogen bomb works by fusing (joining together) the nuclei of atoms of the gas hydrogen. Atomic bombs and hydrogen bombs are the two main kinds of nuclear weapons. The hydrogen bomb is far more powerful and destructive than the atomic bomb. The hydrogen bomb is like a tiny star. It works by the same process—the fusion of hydrogen atoms—that makes the Sun and other stars shine. A nuclear weapon destroys by the power and heat of its blast. The atomic bomb dropped on Japan flattened buildings within 3 miles (5 kilometers) of the blast. Heat from the bomb caused fires and burned everything near the place it exploded. People’s skin was burned as far as 11 miles (18 kilometers) from the blast site. A nuclear weapon also releases harmful radiation. People near the blast can die of radiation sickness even if the bomb doesn’t kill them. People farther from the blast may develop cancer and other illnesses from radiation months and years after the bomb explodes.

THE FUTURE OF NUCLEAR WEAPONS
No one has used a nuclear weapon in war since the United States dropped atomic bombs on Japan in 1945. For some years, countries tested their bombs underground or in remote places. However, test-ban treaties have halted the testing of nuclear weapons. The Cold War ended in the 1990s. It left the United States and Soviet Union with huge numbers of nuclear weapons. Other countries also have built nuclear weapons. The large number of nuclear weapons has produced new fears. What if a terrorist or an unstable government gets hold of a nuclear weapon? This possibility continues to frighten people.

Can you imagine a world without cameras? There would be no photographs in newspapers, books, and magazines, or even on your computer. There would be no school pictures, no snapshots of your summer vacation, no television, and no movies.

It’s hard to imagine, but that’s what the world was like until the mid-1800s. That’s when the first cameras were made.

HOW DO CAMERAS WORK?

A basic camera works a lot like your eyes. Try this: First, close your eyes. Now quickly open and shut them. What did you see? You saw an image, or “picture,” from your surroundings.

A camera does the same thing, but it has a shutter instead of eyelids. When you take a picture, the shutter quickly opens and shuts. While the shutter is open, the camera “sees” an image, much like your eyes. The camera captures this picture.

A film camera catches the picture using chemicals on film. A digital camera captures the image electronically and stores it in memory or on a computer disk. The first popular photographs, called daguerreotypes, were captured on copper plates in the 1840s. Later, pictures were recorded on glass plates. Flexible film, much like we still use today, replaced glass plates in the late 1800s.

Like your eyes, a camera has a lens. A lens is a piece of glass shaped to focus light so the picture will be clear. Some cameras even have automatic focus, just like healthy eyes. If a camera lens is out of focus, the picture will be blurry.

HOW CAMERAS CHANGED THE WORLD

The camera changed the world. Before the camera was invented, people created pictures by painting or drawing. That took time and could be inaccurate.

Around 1840, that all changed. The camera allowed people to keep a visual record of their lives and important events. Suddenly, people could see pictures of faraway places. The camera brought the whole world into people’s homes. Photographs began to influence people’s opinions about the world.

Cameras brought big changes to family life as well. Before the camera, only wealthy people could afford to pay painters to make portraits. Suddenly, ordinary people could afford to have snapshots of themselves and their children or grandchildren.

Later, the motion-picture camera was invented. Thanks to that, we have television and movies.

CAMERAS EVERYWHERE

Today, many people have cameras. Most people use point-and-shoot cameras. A point-and-shoot camera automatically focuses the lens and controls how quickly the shutter opens and closes.

Many banks, stores, and schools use security cameras to watch what people are doing. Cameras on highways show traffic patterns. There are even tiny cameras on some computers and cell phones.

Cameras are important tools for scientists. Doctors use tiny cameras to look inside the human body. Cameras on satellites orbit Earth, taking pictures of weather patterns. Cameras bring us pictures from the deepest oceans, the insides of volcanoes, and even of distant galaxies in space! Cameras are just about everywhere.

Adolf Hitler was a struggling young artist who became a feared dictator. He led his country into a bloody war that killed millions of people.

Hitler rose to power in Germany in the 1930s. He eventually started World War II (1939-1945), a conflict that left Europe in ruins.

HITLER’S CHILDHOOD

Adolf Hitler was born in 1889. He came from a well-to-do family in Austria-Hungary. His father was an important government worker. After his father died, Hitler quit school in the ninth grade. He decided to become an artist but had trouble finding work.

WORLD WAR I

Hilter volunteered for the German army during World War I (1914-1918) and served the whole war. Germany lost the war, and the country suffered terribly. Many Germans became jobless and poor. The people wanted someone to lead them back to glory again. Hitler wanted to be that person.

RISE TO POWER

After the war, Hitler joined the National German Socialists Workers’ Party. Many people called it the Nazi Party, for short. Hitler was an excellent public speaker. He appealed to German pride by constantly speaking about their racial superiority. This was the idea that one type of people are naturally better than others. He blamed other people, especially Jews, for Germany’s problems. His speeches attracted thousands of people who thought Hitler could be a great leader. The Nazi Party grew rapidly.

Hitler ran for political office in Germany and was elected in 1930. Three years later, in January 1933, Hitler became Germany’s chancellor, which was similar to a president. He immediately passed laws giving himself total power. Soon, Hitler had become a dictator. He controlled Germany’s government completely.

Hitler passed laws to get rid of people he did not like. They included his political enemies and Germans who were disabled or Jewish. Many of these people were sent to large camps, where they were held prisoner. Huge numbers of people were killed.

WORLD WAR II

Hitler also began rebuilding Germany’s military. He wanted a powerful army so he could conquer other countries, and eventually take over the world. He started by declaring Germany's union with the neighboring country of Austria. Then he ordered German troops to occupy all of Czechoslovakia. When Hitler’s army invaded Poland in 1939, Great Britain and France declared war on Germany. World War II had begun.

Germany’s mighty army soon captured France and began bombing England. In 1941, Hitler’s armies also invaded the Union of Soviet Socialist Republics (USSR), often called Russia. This turned out to be a big mistake because the German army had trouble fighting in several countries at once.

THE HOLOCAUST

Hitler’s soldiers forced tens of thousands of Jews in Poland into small sections of the cities, known as ghettos. The Jews were not given adequate food, and many of them starved to death. Hitler’s army also sent millions of Jews from Germany and other countries to concentration camps. There, many were killed. The deaths of millions of Jews under Hitler is known as the Holocaust. About one-third of the world’s 18 million Jews died in the Holocaust, one of history’s greatest tragedies.

HITLER’S SUICIDE

The United States entered World War II in December 1941. Slowly, Germany began to lose the war. America and its allies launched the D-Day invasion of western Europe on June 6, 1944. They fought their way through France and into Germany in 1945. Facing defeat, Hitler killed himself. His reign of terror was finally over.

Christopher Columbus tried to take a shortcut, and ended up somewhere he never intended to go. He discovered two continents that people in Europe didn’t even know existed. By crossing the Atlantic Ocean in 1492, Columbus opened contacts between lands and peoples that were unknown to each other.

Columbus’s voyage to the Americas opened an exciting period in history. Animals, plants, and new ideas were exchanged between continents. But it also caused terrible tragedy. Millions of Native Americans died as Europeans rushed to take land and riches for themselves.

MASTER SAILOR

Christopher Columbus was born in 1451 in Genoa, Italy. He became a sailor at the age of 14. In 1476, he was shipwrecked off the coast of Portugal. Portugal was Europe’s top seafaring nation at that time. Columbus settled there.

Columbus studied geography and navigation, the science of figuring out where things are on Earth’s surface. He became a master sailor. He met explorers who had sailed along the coast of Africa seeking an eastward sea route to the rich lands of Asia. Europeans called these lands “the Indies.” Europeans wanted to bring gold and other treasures from the Indies back to Europe.

DARING DREAM

Columbus began to think about a wonderful adventure, which he called the “Enterprise of the Indies.” He dreamed of reaching the Indies by sailing west! This was not a new idea, but no one had ever managed to make the voyage. Columbus thought the trip to the Indies west across the ocean would be much shorter than sailing around Africa.

Columbus had high hopes, but no money. Who would pay for his expedition? He asked the king of Portugal, but the king refused. Columbus didn’t give up. He went to the rulers of Portugal’s neighbor, Spain. At first they also refused. Eventually, however, the Spanish king and queen agreed to provide three small ships—the Pinta, the Niña, and the Santa María. They also paid for crews and supplies for the voyage.

HISTORIC VOYAGE

Columbus sailed from Palos, Spain, on August 3, 1492. He stopped at the Canary Islands southwest of Spain, then headed west into unknown seas. He had no idea what lay ahead, but he had faith in his sailing skills and his bold idea. A swift current carried his ships along, and on October 12, the crew sighted the islands of the Bahamas. Columbus thought he had reached Asia. He called the islands the Indies.

Columbus was greeted by the Arawak people who lived on the islands. They offered food, but had only a little gold. Columbus was disappointed not to find Asian treasures, but still felt sure he had reached Japan in Asia. He spent two months exploring, then headed home. One of his ships sank in a storm, but back in Spain he was hailed as a hero. The king and queen offered rich rewards and made him “Admiral of the Ocean Seas.”

THREE FAILURES

Columbus made three more voyages to America. None went well. He was a skillful sailor, but his greed and stubbornness made him a bad leader and created enemies.

During his second voyage (1493-1496), Columbus claimed land for Spanish settlements. He fought against Caribbean peoples who lived on the land he claimed and forced them to work as slaves.

On the third voyage (1498-1500), Columbus quarreled with Spanish settlers so violently that he was sent back to Europe as a prisoner in chains.

On his fourth and final voyage (1502-1504), Columbus was marooned on an island for more than a year. He had to be rescued. He was very ill by the time he returned home to Spain.

AN EXTRAORDINARY EXPLORER

Columbus died in 1506. He quarreled with the king and queen right up until his death. He wanted authority over Spanish colonies and a larger share of the riches that were brought back from America. It was a sad end to an extraordinary career that still shapes our lives today. When Columbus crossed the Atlantic, he changed the world forever.

Pretend you are an alien explorer from outer space looking for life on other planets. Your spaceship flies into a group of stars that looks like a gigantic whirlpool. The whirlpool is the Milky Way Galaxy.

You head for a star with nine planets in one arm of the Milky Way. The third planet from the star is a beautiful blue, white, and green ball. This planet looks like it has life. The name of this planet is Earth.

HOW DOES EARTH MOVE IN SPACE?

Earth spins like a top on its axis. Earth’s axis is an imaginary line that goes through Earth from the North Pole to the South Pole. Earth’s axis is slightly tipped, like a spinning top leaning to one side.

Earth travels around the Sun at about 67,000 miles per hour (about 107,000 kilometers per hour). One year is one trip around the Sun. Earth’s path around the Sun is slightly oval-shaped. This oval shape causes Earth’s distance from the Sun to change during the year.

WHAT MAKES DAY AND NIGHT?

The Sun seems to rise in the morning, cross the sky during the day, and set at night. However, the Sun does not actually move around Earth. Earth’s turning on its axis makes it look as if the Sun is moving.

Earth makes a complete turn on its axis every 24 hours. As Earth turns, half of the planet faces the Sun, and the other half faces away. It is daytime on the half facing the Sun. It is night on the half facing away from the Sun.

WHY ARE THERE SEASONS?

Earth has seasons because of the tilt of its axis. For part of the year, the top half of Earth is tipped toward the Sun. The top half of Earth is called the Northern Hemisphere. During another part of the year, the bottom half of Earth is tipped toward the Sun. The bottom half is called the Southern Hemisphere. It is summer in the half that is tipped toward the Sun. It is winter in the half tipped away. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere. During spring and fall the hemispheres are tipped neither toward nor away from the Sun.

The equator is an imaginary line around Earth’s middle. The farther you are from the equator, the greater the difference in temperature between seasons. The equator never tips far from the Sun. Near the equator it is warm enough to go swimming all year long. The average temperature barely changes from month to month. In Alaska, far from the equator, the average temperature in January can be more than 60 degrees colder than it is in July.

WHY IS THERE LIFE ON EARTH?

Earth has just the right conditions for life. It is not too hot or too cold. Earth has lots of liquid water and an atmosphere (gases) that can support life.

The first kinds of life may have appeared on Earth 3.8 billion (3,800,000,000) years ago. Several times during Earth’s history, almost all life went extinct, or disappeared. Each time, some life forms survived. The survivors spread all over the planet. Dinosaurs appeared about 230 million years ago. Dinosaurs went extinct about 65 million years ago. Scientists believe that modern humans appeared about 130,000 years ago.

WHAT IS THE INSIDE OF EARTH LIKE?

Earth is made of layers. The top layer is called the crust. It is made of hard rock and soil.

More than 70 percent of Earth’s crust is covered with water. Most of the water is salt water in the ocean. Pieces of dry land called continents rise above the ocean. The part of Earth’s crust under the ocean is called the seafloor.

Under the crust is a layer of partly melted rock called the mantle. Under the mantle is Earth’s core. The core is mostly iron. The outer part of the core is liquid metal. The inside of the core is solid metal. Scientists believe that the liquid metal makes Earth a giant magnet and creates Earth’s magnetic field.

Earth’s crust is made of gigantic slabs of rock called plates that move over the mantle. Plates crash together to make mountains. They pull apart and let red-hot rock ooze up from inside Earth to make new crust.

HOW DID EARTH FORM?

Scientists think that Earth and the rest of the solar system formed from a spinning cloud of gas and dust. Gravity pulled most of the gas and dust together to form the Sun. Some leftover gas and dust formed Earth and the other planets. Scientists think that Earth and the Moon formed about 4.6 billion years ago.