X Rays

Imagine that you could see right through your own skin. You could see the bones inside your body. You could watch food go down your throat when you swallow it. Imagine looking inside someone’s suitcase to see what’s inside. Does that sound impossible? Not when you know about X rays!

WHAT ARE X RAYS?
X rays are very powerful light rays that your eyes can’t detect. These light rays can slip through objects that visible light bounces off. We use X rays as a powerful tool to detect and discover things our eyes can’t see.

HOW WERE X RAYS DISCOVERED?
X rays were discovered by accident. In 1895, a man named Wilhelm Roentgen was experimenting with electricity in vacuum tubes in a black cardboard box. He noticed that a special screen he had nearby glowed when electricity went through the tubes. He experimented more and determined that invisible light rays from the tubes caused the screen to glow. These rays went right through the cardboard box! He named the invisible light rays he had found X rays.

WHAT ARE X RAYS USED FOR?
Just a few years after X rays were discovered, doctors were already using them to find bullets inside people who had been shot. Doctors later began to use X rays to find out if people are sick or have broken bones. Dentists use X rays to check up on people’s teeth. An X-ray device called a CAT scan rotates around a person and creates a 3-D picture of the person’s insides on a screen. This device gives doctors clear views inside any part of the person’s body. Scientists who study matter and energy often use X rays in their research. X rays help them see what things are made of. Many chemical elements were discovered using X rays. Industries use X rays to test products and materials for flaws such as cracks in an airplane wing. X rays are also used to tell whether gems and works of art are real or fake. Border guards use X rays to look inside cars and containers. The X rays can find goods that are being smuggled from one country to another. Airports use low energy X rays to see inside luggage and check for dangerous items.

DOCTORS’ X RAYS
When a doctor takes an X ray of you, the X-ray machine shoots X rays at you. Most of the rays go through you and into a special film, which catches them. Some of the X rays that hit your bones, however, don’t make it through you. Bones absorb X rays more than other parts of your body. Because X rays absorbed by your bones never make it to the film, lighter areas appear on the film where your bones are! These lighter areas provide a picture of the bones. X rays can be harmful. Doctors use X rays to kill cells that are harmful to people, such as cancer tumors. Because too many X rays can be harmful, doctors warn that X rays should be used only
when necessary.

Ships

Every day, huge ships made of steel cross the oceans and travel the world’s great rivers and lakes. Powerful engines turn propellers that make the ships go. Ships transport people and goods to all parts of the world. Ships are very important to the way we live. Ships carry oil that is made into gasoline for our cars. They bring in much ofthe food we eat and the clothes we wear. They carry computers, furniture, and televisions for our homes. Look around you. Many of the things you see traveled to where you are on a ship.

THE PARTS OF A SHIP
Ships may look very different from each other, but they all have the same basic parts. All ships float in water. The part that floats is called the hull. Inside the hull there are decks. Decks are like the floors in a building. You can go up and down from one deck to another.

HOW SHIPS MOVE THROUGH WATER
The front of a ship is called the bow. The back is called the stern. Attached to the stern is a wooden or metal plate called the rudder. A steering wheel or a stick called a tiller makes the rudder swing back and forth. Moving the rudder makes the ship turn. Some ships use sails to move. Sails are big sheets of fabric. The sails hang from a long pole called a mast. Ships with sails use the energy of blowing wind to move through the water. Most modern ships have engines that burn fuel. Engines make power to turn propellers at the stern. Propellers make ships go
through the water.

THE AGE OF SAILING SHIPS
By about 5,000 years ago, the Egyptians were building some of the first sailing ships. They made them by tying bundles of reeds to a wooden frame. The ships carried cargo and had one or two square sails. The best ancient shipbuilders were the Phoenicians. They made cargo ships and warships called galleys. Galleys had sails and many oars. The ancient Greeks fought with the Phoenicians. The Greeks added a big spike to the front of their galleys. They used the spike to ram into Phoenician ships. In China and other parts of Asia, builders made cargo ships called junks. Junks had a flat bottom, a square bow, and a rudder. The sails had pieces of bamboo in them to make them stiffer. Arab builders began to use triangular sails called lateens. A ship with lateen sails could sail almost directly into the wind. In the 1200s, Europeans began building ships with three masts and many square and triangular sails. These ships were called full-rigged ships, or square-riggers. Starting in the 1400s, European explorers set off on voyages in these ships to faraway parts of the world. Christopher Columbus, Vasco da Gama, and other explorers used square-rigged ships.

In the 1600s, the Spanish built huge ships called galleons. In the 1700s and 1800s, the British built big sailing ships that they used to fight sea battles. The fastest sailing cargo ships were the clipper ships of the mid-1800s. They had sleek, narrow hulls and as many as six sails on each tall mast. MODERN SHIPS During the 1800s, iron and steel hulls replaced wooden hulls. New types of engines were also developed. For the first time, ships could move without wind or human-powered oars. Steam engines fueled by coal replaced sails. Later, engines that used oil as a fuel replaced steam engines. Today, most ships have steel hulls and are driven by powerful motors that turn big propellers.

CARGO SHIPS
There are many kinds of cargo ships. Container ships carry cargo in huge boxes the size of railroad cars. Oil tankers and supertankers carry oil in their hulls. Freighters transport tons of coal, grain, and ore.

PASSENGER SHIPS
There were no passenger ships in ancient times. Travelers had to look for space on a cargo ship. Most passengers slept wherever they could find a spot on the deck. After Europeans learned about the Americas and Australia, settlers wanted to move to these new lands. Full-rigged ships carried passengers along with cargo. It was not very comfortable traveling on those wooden sailing ships.

By the mid-1800s, shipping companies began to offer regular passenger service. Companies competed with each other for passengers. They built luxurious ocean liners that could cross the
Atlantic Ocean in just a few days. In the 1950s, airplanes became more popular than ships for traveling over oceans. Today, most passenger ships are cruise ships. You can take a vacation aboard big cruise ships.

NAVY SHIPS
For many years, battleships were the biggest warships. They were used in World War I and World War II. Today, aircraft carriers are the biggest warships. The largest carriers can hold 85 airplanes. They have crews of more than 5,500 people. Modern navies have many other kinds of ships. Submarines are ships that can dive underwater. Some submarines carry missiles toattack enemy ships. Cruisers escort and defend aircraft carriers from attack by planes and submarines. Destroyers defend carriers and merchant ships from air and submarine attacks. Frigates escort
and defend ships from submarines.

THE NEWEST SHIPS
Shipbuilders are looking for ways to build big ships that go faster and carry more cargo. They are looking for new hull shapes that go faster in the water. They are also looking for better engines. Water jet engines may replace propellers. A jet boat engine works by shooting out water, just as a jet plane engine shoots out air.

Muscles

How strong are you? Can your legs run fast? Can your arms lift heavy books? Your muscles make your legs run fast. Your muscles let your arms lift heavy books. Without muscles, you wouldn’t be able to move at all!

HOW DO MUSCLES WORK?
Muscles work by tightening and loosening. Tightening is called contraction. Loosening is called relaxing. Your nerves tell muscles when to contract. Suppose you see a cookie on a table. You want to pick up that cookie and eat it. Your brain sends out a signal. Nerves carry the signal from your brain to your arm and hand muscles. The signal tells muscles in your arm to contract. Your arm reaches out for the cookie. Then the signal tells muscles in your hand to contract and grab the cookie. Muscles in your arm contract to bring the cookie to your mouth. Your hand pops the cookie into your mouth. Your jaw muscles contract and relax so you can chew the cookie. Yum.

WHAT ARE MUSCLES MADE OF?
Muscles are bundles of thin strands called fibers. The muscle fibers are made of substances called proteins. There are two types of muscle fibers. The two types are slow twitch and fast twitch. Your fast-twitch muscle fibers contract rapidly. These fibers give you bursts of power. When you suddenly jump or run fast while playing tag, your fast-twitch muscle fibers are hard
at work. Slow-twitch muscle fibers allow you to keep doing exercises. They give you endurance. When you run a long way, your slow-twitch muscle fibers are doing most of the work. Some kinds of muscle have both fast-twitch and slow-twitch fibers.

ARE THERE DIFFERENT KINDS OF MUSCLES?
There are three types of muscle called skeletal, smooth, and cardiac. Each kind of muscle has a different job to do. Your skeletal muscles are attached right to your bones. These muscles contract and relax to make your bones move. These are the kind of muscles you use to run or swim or reach for cookies. You usually have to think about making these muscles contract. Smooth muscles move automatically. You do not have to think about your smooth muscles to make them work. Your digestive system is surrounded by smooth muscle. Smooth muscles push
food through your digestive system. Your biggest blood vessels are surrounded by smooth muscle to make them stronger. Your cardiac muscle makes your heart beat. You do not have to
think about moving your cardiac muscle. Your cardiac muscle contracts automatically.

WHAT MAKES MUSCLES STRONG?
Exercise makes muscles stronger. Lifting heavier and heavier weights makes your skeletal muscles stronger. Many people lift weights to make their arm and leg muscles stronger. A gym
teacher or special trainer can show you how to lift weights safely. Running, walking, swimming, and jumping rope are exercises that can make your heart muscle stronger. Any exercise that makes your heart beat faster makes your heart muscle stronger. Muscles can get weaker if they are not used. Using your muscles every day keeps them strong and healthy.

Helicopters

Helicopters can fly straight up. They can fly forward, sideways, and backward. They can even hover in one place. An airplane must speed down a long runway to take off and land. Wings hold an airplane in the air. Helicopters do not need runways, and they do not have wings.

WHAT HOLDS A HELICOPTER IN THE AIR?
Big blades on top of a helicopter keep it in the air. The blades are a little like fan blades. The blades spin very fast. Wind blowing down from the whirling blades holds a helicopter up. The blades also control the direction in which the helicopter flies. The blades make a loud chop-chop-chop noise as they turn. The noise caused people to nickname helicopters “choppers.” Helicopters cannot fly as fast as an airplane. The fastest helicopters go about 200 miles per hour (320 kilometers per hour). They also cannot go as far as an airplane. Helicopters burn a lot of fuel.

HOW MANY BLADES DO HELICOPTERS HAVE?
The blades of a helicopter are called the rotor. Some rotors have two blades. Some rotors have three or four blades. Some big helicopters have rotors with eight blades. Big helicopters sometimes have two rotors on top. A long metal tail sticks out from the back of most helicopters.
These helicopters have a small rotor on the tail. The tail rotor blows air sideways instead of down. It helps the helicopter steer.

HOW DO YOU FLY A HELICOPTER?
A helicopter has a cockpit just like an airplane. The controls are in the cockpit. A helicopter has two control sticks, or levers. It has two pedals on the floor. When you’re flying a helicopter, you push or pull on the stick on your left side to make the blades tilt. Tilting the blades makes the helicopter go up or down. A grip on this stick controls the speed. You twist the grip to make the helicopter go faster or slower. The other lever is between your knees. You move this stick around to make the helicopter fly forward, backward, or sideways. The pedals on the floor control the tail rotor. You step on the pedals to turn the helicopter. Pushing the left pedal makes the helicopter turn left. Pushing the right pedal makes the helicopter turn right. HOW DO WE

USE HELICOPTERS?
Helicopters can go places that are hard to reach. They can go places where airplanes cannot land, or where there are no roads for cars or trucks. Helicopters can rush injured people from a car accident to a hospital. They can rescue people from the tops of burning buildings. They can pluck them from trees in the middle of raging floods. They can lift people from the decks of sinking ships at sea. Military helicopters are important in war. They carry troops to battle. They carry wounded soldiers to hospitals. They can even shoot missiles. Helicopters do other kinds of work. Reporters can fly in helicopters to cover news stories. Police use helicopters to chase suspected criminals. Farmers can use helicopters to spray their fields. Sometimes helicopters work on construction. The carry heavy parts to the tops of buildings.

WHO INVENTED THE HELICOPTER?
People imagined machines like helicopters hundreds of years ago. The ancient Chinese made a spinning top that could rise up into the air. The Italian artist and inventor Leonardo da Vinci drew a design for a machine like a helicopter in 1480. The first helicopters flew in France in 1907. They were hard to control. People kept trying to build better helicopters. Finally, a Russian-born American engineer named Igor Sikorsky made a workable helicopter. It was the first helicopter with a tail rotor. He flew this helicopter in 1939. Most helicopters today are like the helicopter that Sikorsky built. Inventors and engineers are still working to make bigger and better helicopters.

Gold

“Gold! Gold! Gold!” screamed the newspaper headlines. “Gold discovered in California!” That was in 1848. The news brought 100,000 people rushing to California. They came seeking the yellow metal that could make them rich. People since ancient times have used gold for jewelry and money. They used it in religious objects and works of art. Wars have been fought over gold.And sometimes, as in California, gold changed the course of history.

WHY IS GOLD VALUABLE?
Gold is unusual among metals. It does not rust or tarnish (grow dull and discolored). Gold coins recovered from sunken treasures are still as shiny as when they sank. Gold is soft enough to be easily shaped into jewelry and other items. An ounce (31 grams) of gold can be hammered into a sheet 16 feet (5 meters) on each side. It can be stretched into a wire 62 miles (100 kilometers) long. People find gold beautiful. And it is rare. All the gold in the world would fit in a cube 65 feet (20 meters) on each side. Because it is so rare, its value doesn’t change much from one year to the next. In ancient times, people could easily carry a lot of wealth in the form of a small bag of gold.

MINING GOLD
The easiest way to mine gold is with a pan. You fill the pan with sand or gravel that contains tiny bits of gold. Then you swirl the pan under a gentle stream of water. The lighter gravel or sand gradually washes out with the water. The heavier gold particles collect at the bottom of the pan. Gold is so heavy that it doesn’t take many tiny flakes to make an ounce. Today, gold is most often mined by digging underground with machines. Rock that contains gold is treated with chemicals to separate out the gold. Nuggets of solid gold are quite rare. The largest nugget ever found weighed about 130 pounds (59 kilograms). It was found in Australia in 1869. About two-thirds of all gold mined today comes from South Africa.

HOW IS GOLD USED?
Gold is used for many things besides coins, decorations, and jewelry. Gold conducts electricity very well. It is used in tiny electrical circuits. There are very small amounts of gold in your computer. Gold is also used to protect tall buildings and spaceships from the Sun’s heat. The Sun's rays bounce off even a thin coating of gold.Gold-coated mirrors are used in telescopes. Dentists use gold for tooth fillings. Gold is even used in medicine, to treat cancer and
arthritis.

GREEDY FOR GOLD
Gold has always made people who controlled it wealthy. Folktales of many peoples tell of greed for gold. The ancient tale of King Midas tells of a greedy king. Midas wished that everything he touched would turn to gold. But he was sorry when his wish was granted. He could not eat because his food and water also turned to gold. In the 1500s, the Spanish conquered Mexico and Peru while searching for gold. They brought back tons of gold looted from native peoples. The peoples of those regions had never considered gold very valuable. The discovery of gold in parts of western America and Australia brought in thousands of people seeking quick fortunes. Many stayed on and settled those regions.

Photosynthesis

Plants need sunlight. Houseplants lean toward the Sun, and if they do not get enough light they wither and die. Plants use sunlight to make their food. This process is called photosynthesis.
Photosynthesis is a scientific word made up from Greek words. These words mean “putting things together using light.” Inside plants’ leaves, light causes air and water to combine to make new chemicals. These chemicals are food for the plants.

FOOD FACTORIES
In most plants, photosynthesis takes place mainly in the leaves. Like other living things, plants are made up of tiny cells. The cells in a plant’s leaves contain even smaller, disc-shaped parts called chloroplasts. Chloroplasts are the food factories where photosynthesis happens. A leaf the size of your little fingernail contains more than 10 million of them. Chloroplasts contain a chemical called chlorophyll, which is bright green. Chlorophyll gives plants their green color and makes photosynthesis work.

THREE INGREDIENTS
For photosynthesis to work, the chloroplasts need to collect three ingredients: sunlight, air, and water. Sunlight shines on the leaf, and the green chlorophyll inside the chloroplasts soaks it up. Air enters the leaf through tiny holes in the leaf’s surface, called stomata. Water is sucked from the ground by the plant’s roots. It travels through tubes in the stem or trunk to the leaves. When all three ingredients are present inside the chloroplasts, a chemical reaction takes place. The reaction takes place between a gas in air called carbon dioxide and hydrogen, a part of water. Sunlight causes these two to combine and make new chemicals called carbohydrates. Carbohydrates are plant food. Plants use these chemicals to live and grow.

THE KEY TO LIFE
Photosynthesis is not just important for plants—it is the key to lifefor all of us. Plants use photosynthesis to make food. We eat the leaves, roots, fruits, and seeds of plants. Spinach and lettuce are leaves. Potatoes and carrots are roots. Tomatoes and apples are fruits. Nuts are seeds. If you eat beef, lamb, or other meat, you still depend on plants. Meat comes from cows, sheep, and other animals that feed on plants. In this way, energy from the Sun is passed on through all the different living things on Earth. If it were not for photosynthesis, plants would not grow. There would be nothing for animals to eat, so they would not exist either.

BREATHABLE AIR
Photosynthesis also produces a gas, oxygen. Plants release oxygen into the air. Humans and other animals need oxygen to live. We breathe in the oxygen produced by plants during photosynthesis.

Eyes and Vision

Think about all the things you do with your eyes. You watch TV and read books. You surf the Internet. You keep your eye on the ball when you play sports. You see your family and friends. Your eyes are your windows on the world. Your eyes are like cameras that focus on what is in front of you. Your eyes work together with your brain to create a picture of the world. The process of creating the picture starts when light rays enter your eyeball.

LOOKING AT YOUR EYEBALL
Look at one of your eyes in a mirror. Your eyeball is round. The inside of your eye is a transparent (see-through), jellylike material called the vitreous humor. The vitreous humor gives your eyeball its shape. You can’t see much of the vitreous humor because it is surrounded by an outer part, or wall. The wall of your eyeball is made up of outer, middle, and inner layers.

THE OUTER WALL
The outside layer of your eye is a protective coating called the sclera and the cornea. The sclera is the white part of your eye. The cornea is clear and goes over the center of your eye, the part you look through. Light rays enter your eye through your cornea.

THE MIDDLE WALL
The middle layer of the wall has three parts called the iris, the ciliary body, and the choroid. What color are your eyes? The color comes from your iris. Your iris is in the center of your eye. It can be shades of brown or blue. The black circle in the center of your iris is called the pupil. It gets bigger or smaller to control how much light comes into your eye. The ciliary body goes around your iris and connects to the lens of your eye. Muscles in the ciliary body pull on the lens to focus it. The blood vessels that bring blood to your eye are also part of the middle wall. These blood vessels are called the choroid.

THE INNER WALL
The inside layer of your eyeball wall is called the retina. Your cornea and lens focus light rays on your retina just as a camera lens focuses light rays on film. They bend light rays coming into your eye so that they will strike the center of the retina called the macula lutea. This is where you have your sharpest vision.

RODS AND CONES
Your retina has millions of light-sensitive cells called rods and cones. These cells pick up the tiniest dot of light that gets to your retina. There are bits of colored material called pigment in the rods and cones. Pigment in the rods lets you see shades of gray and helps you see at night. Pigment in the cones lets you see colors. The rods and cones change light rays into electrical signals. Nerves pick up these signals and carry them to your optic nerve.

OPTIC NERVES
An optic nerve leads from each of your eyes to your brain. Each eye picks up slightly different images. When these images get put together, you can see in 3-D. You have depth perception that lets you tell how far away things are. Your optic nerves are like big cables that carry all the signals to a special part of your brain. This “media center” in your brain makes a picture of the world. It gives you sight.

EYE PROTECTION
Your eyeballs are set into two holes in your skull called eye sockets. The bones of your skull protect your eyes. Muscles let you turn your eyes in their sockets. Eyelids and eyelashes also protect your eyes. You can close your eyelids to keep dust or bright light out. Your eyelashes are a fringe of short hairs on each eyelid. They screen out dust when your eyelids are partly closed.
Inside the eyelid is a thin layer called the conjunctiva. It covers part of the sclera. Each eye also has a tear gland that gives off salty liquid to wash small particles out of your eye.

VISION PROBLEMS
Do you wear contacts or glasses? If not, you probably know someone who does. Many people need contacts or glasses becausethey are nearsighted. Things far away look blurry. Light rays focus in front of the retina because the eyeball is too long. Some people have the opposite problem. They are farsighted and can’t see close-up things very well. In farsightedness, light rays focus behind the retina because the eyeball is too short. Astigmatism is another vision problem. A person has an astigmatism when their cornea is unevenly curved. Older people sometimes need reading glasses because the muscles in their eyes can no longer focus on things that are nearby.

EYE DISEASES
Diseases strike different parts of the eye. A sty is an infection of the eyelid. Conjunctivitis, or pinkeye, is an infection of the thin layers covering the eyelid and outer eyeball. Many eye diseases are most common in older people. Sometimes the lens of the eye gets cloudy over time. This condition is called cataracts. Retinas can get detached (come loose) from the back of the eye and cause blindness. Glaucoma occurs when fluid gets trapped between the cornea and the lens and puts pressure on the eye. A problem called macular degeneration affects the center of the retina. It can cause blindness in older people. There are many treatments for eye diseases.
Doctors treat infections with drugs. They use laser beams to weld detached retinas back into place. Surgeons can replace clouded lenses with clear plastic ones. They can also replace diseased corneas. It is very important to protect your eyes. Get regular eye examinations. Wear eye protectors when doing dangerous work or playing rough sports. Wear sunglasses that protect against harmful rays from the Sun. Your eyes are too important to take chances
with!

Communism

What if nobody was rich and nobody was poor? Suppose valuable things like land and factories belonged to everyone. Imagine a world in which everyone worked, but no one got paid. Why? Because everything would be free. These are among the basic ideas of a movement called communism.

HOW WAS COMMUNISM BORN?
The ideas of communism became popular in the early 1800s. At that time, the first big factories were forming. Most workers in these factories earned low wages. Meanwhile, the factory owners were getting rich. This made many workers angry. Two Germans, Karl Marx and Friedrich Engels, turned this anger into a political movement. They announced the goals of this movement in their 1848 book, The Communist Manifesto. Marx and Engels said that all through history, different classes (groups of people) had been at war. They said the warring classes of their time were the owners of businesses and the people who worked for them. Marx and Engels called for a world in which the workers themselves owned all businesses. They said that if workers were in charge, everybody would get what they needed to live good lives. Marx explained these ideas further in a book called Das Kapital.

THE SPREAD OF COMMUNISM
The ideas of Marx and Engels spread through Europe. In Russia, a man named Vladimir Lenin said workers needed a small, organized group to lead a revolution on their behalf. To do this, he formed the Bolshevik Party (later called the Communist Party). In 1917, a revolution overthrew the emperor of Russia. In the fighting that followed, Lenin’s party took over. Soon, his communist government owned all the land, factories, stores, and businesses in Russia. The communists in Russia conquered some neighboring countries, too. They called their new empire the Union of Soviet Socialist Republics (USSR), or Soviet Union for short. After Lenin died, Joseph Stalin took power in the USSR. He used brutal force to make the Soviet Union into a communist country. Stalin killed, imprisoned, and tortured millions of people. He built a country in which everyone worked for the government. The government made almost all decisions for the people.

HOW DID COMMUNISM SPREAD?
In World War II, the Soviet Union fought on the side that won. When the war ended in 1945, the Soviet Union gained control over most of the countries in Eastern Europe. In China, meanwhile, a communist leader named Mao Zedong was rising in power. He seized control of his country in 1949. China, with its huge population, became the biggest communist country of all.

WHY DID COMMUNISM LOSE STRENGTH?
Many noncommunist countries fought to keep communism from spreading. They feared that communism would keep expanding and threaten noncommunist countries.The United States led the fight against communism, while the Soviet Union supported communist movements around the world. This struggle is known as the Cold War. It lasted for more than 40 years. By the 1980s, communism was failing. Under communism, the Soviet Union could not produce enough goods for its people. It grew poor. Eastern European countries began to break away from
the Soviet Union. In 1991, the Soviet Union itself broke apart into 15 separate countries. None of these countries have communist governments today.
In China, the Communist Party still holds power. But China is allowing privately owned businesses to grow again. Other communist countries today include Cuba, Vietnam, Laos, and North Korea. As a world movement, communism appears to be losing its
appeal.

Color

The world is full of beautiful color. You see a violet dress, a blue car, a yellow flower, and a green tree. The colored leaves on trees in autumn mean that winter is coming. All kinds of colors are everywhere.

COLOR COMES FROM LIGHT
White light, including sunlight and light from a light bulb, is actually made of all the colors of the rainbow. Have you ever seen sunlight that hits a piece of crystal? Rays of blue, purple, orange, yellow, and red seem to shoot out from the crystal in all directions. The crystal spreads the colors of light apart a bit so you can see them separately. Scientists show the colors of light in a bar called a spectrum. A rainbow is a spectrum. Its colors go from red through orange, yellow, green, blue, indigo, and violet. Violet is purple and indigo is a deep purplish-blue. It’s easy to remember the colors of the spectrum because the first letter of each color makes up a name:
Roy G. Biv.

HOW DO WE SEE COLOR?
White light, such as sunlight or light from a light bulb, lets you see things. Dresses, cars, and all other things we see absorb (soak up) some of the colors of white light. The colors that don’t get absorbed bounce off of things. Red light bounces off a red dress. The dress soaks up other colors. Your eyes see the red light but not the other colors. Your eyes send this message to your brain. Your brain tells you that you are seeing a dress that looks red. Something very special happens when you see a red dress or green grass. Objects themselves don’t actually have color. What they have is the ability to reflect (bounce back) different types of light. When this reflected light enters your eyes, your brain interprets the different types of light as color. Your eyes and brain work together to translate the different types of light into different colors. COLORS OF PAINT Artists call three special colors the primary colors of paint. These colors are magenta (purplish-pink), yellow, and cyan (light greenish-blue). You can make other colors of paint by mixing the primary colors together. But you cannot mix other colors of paint to make a primary color. Suppose you want to paint a picture of an apple tree. You can make whatever colors you want to use with just four jars of paint: magenta, yellow, cyan, and white. You mix yellow and cyan to make green paint for the leaves. For the apples, mix magenta and yellow paint to make the color red. To paint the sky light blue, you must use some white paint. White makes other colors lighter. Mix magenta and cyan to make a deep blue. Then add some white paint to the blue paint until the blue becomes light enough for the sky. White paint mixed with blue or another color is called a tint. A light blue tint will make a color like a clear sky. If you mix all three colors together you get black paint. You can make a color darker by mixing it with black paint. Colors mixed with black paint are called shades. When you mix black and white
together, you get gray. By mixing together different amounts of magenta, yellow, cyan, and white paint, you can make lots of different colors. Everything you need to make beautiful paintings.

Cameras

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. Acamera 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.

Brain and Nervous System

What kind of supercomputer can write stories, do math problems, draw pictures, play games, see through eyes, hear someone talking, talk back, and network with devices that make snacks in the microwave oven? Your brain and nervous system can do all these things. Do you think a computer will ever be as powerful as your brain? You think with your brain. Your brain also sends signals through a network called your nervous system. It tells your legs to walk and run. It tells your hands and arms to put popcorn in the microwave. You don’t even have to think about many of the things your brain does. Your brain tells your heart to beat. It tells your lungs to breathe in and out, even when you are sleeping. Your brain also controls your feelings. Such feelings as joy, sadness, love, anger, and fear all come from your brain.

WHAT IS MY BRAIN MADE OF?
Your brain is made of about 100 billion nerve cells. It looks like a lump of pinkish-gray jelly. The surface of the brain is wrinkled, and deep grooves divide it into sections. A network of blood vessels brings oxygen and food to your brain cells and carries away wastes. Your brain is protected by bone called your skull. Liquid and skinlike tissues also protect your brain. When you were born, your brain weighed about  pounds (about 0.35 kilograms). Your brain keeps on growing while you grow up. By the time you reach the age of 20, your brain will weigh about 3
pounds (1.3 kilograms). Your brain has three main parts. The parts are called the cerebrum, the cerebellum, and the brain stem. The cerebrum makes up the largest part of the brain. The cerebellum is underneath the back part of the cerebrum. The brain stem connects with the spinal cord at the bottom of the brain. Your cerebrum and cerebellum are divided into two parts. These parts are called the right brain and the left brain. The right side of your brain controls the left side of your body. The left side of your brain controls the right side of your body. Nerves from the right and left side of your body cross over when they enter your brain.

WHAT DOES THE CEREBRUM DO?
Your cerebrum makes up most of your brain. Your cerebrum solves problems and makes wishes. All of your thinking goes on in your cerebrum. Speech, language, and emotions come from your cerebrum, especially your cerebral cortex. The cerebral cortex is the outer part of the cerebrum. Your cerebrum also gets signals from your senses. Nerves carry the signals. Nerves from your eyes and ears go to parts of the cerebrum that let you see and hear. Nerves carry signals to your cerebrum that let you feel, smell, and taste. Your cerebrum sends messages out along nerves. The messages tell your legs to walk or run. They tell your arm and hand to wave when you see a friend across the street.

WHAT DOES THE CEREBELLUM DO?
Your cerebellum coordinates and fine-tunes your body movements. Your cerebrum might tell your hands and arms to hit a baseball. Your cerebellum controls how you swing the bat and make contact with the ball.
Your cerebellum helps your fingers play the piano, guitar, or violin. It helps you keep your balance when you run, jump rope, or walk along a curb.

WHAT DOES THE BRAIN STEM DO?
Your brain stem takes care of all the things that you do but don’t need to think about doing. It keeps your heart pumping blood. It keeps your lungs breathing air. It makes your eyes blink. It pulls your hand back really fast if you touch a hot pot on the stove.

WHAT IS THE NERVOUS SYSTEM?
Your nervous system consists of the brain, spinal cord, and nerves that run throughout your body. The nervous system carries messages to your muscles and organs. These messages tell your body what to do. Your spinal cord is made of bundles of nerves. It starts in your neck and goes down your back. Nerves go out from the spinal cord to other parts of your body. Nerves from the spinal cord extend to the tips of your fingers and toes. Your spine, or backbone, protects your spinal cord.

Bones and Skeleton

Squeeze your arm. The outside of your arm is soft, but there is a hard part inside. The hard part is a bone. There are bones in your arms and in your legs. Bones go up the middle of your back. They go around your chest. All of your bones together make up your skeleton. Your skeleton holds your body up. It gives your body its shape. Bones do many other important jobs in your body.

WHAT DO BONES DO?
Many bones protect the soft parts inside your body. Skull bones around your head protect your brain. Rib bones make a cage around your chest. Your rib cage protects your lungs and heart. Muscles hook on to bones. Muscles pull on your bones to make them move. Muscles and bones together let you stand, sit, and walk around.
Blood is made in the center of bones. The center of a bone is filled with bone marrow. Bone marrow is soft. Red and white blood cells are made by bone marrow. Red blood cells carry oxygen to all parts of your body. White blood cells help your body fight germs. Three tiny bones help you hear. The three bones are deep inside your ears. One of these bones is called the stirrup bone. It is the smallest bone in your body.

WHAT ARE BONES MADE OF?
There are two kinds of bone. One kind is called compact bone and the other is called spongy bone. Compact bone is the hard and smooth part on the outside of a bone. The long bones in your arms and legs have lots of compact bone. Spongy bone usually lies under the compact bone. Spongy bone is at the ends of arm and leg bones as well. Bones of the pelvis (hipbone), ribs, breastbone, backbone, and skull also contain spongy bone. Your skeleton also contains cartilage. Cartilage is like bone but softer. It bends easily. There is cartilage in body parts that must be
tough but able to bend. There is cartilage in the tip of your nose and in the outer part of your ear.

WHAT ARE JOINTS?
Joints are the places where two or more bones meet. Most bones are tied together at joints by tough bands called ligaments. Different kinds of joints let you move in different ways. Move your lower arm up and down. Keep your upper arm still. The joint that joins your upper and lower arm is called the elbow. Your elbow works like a hinge. It lets you move your lower arm, but only up and down. Now swing your arm all around from your shoulder. A joint in your shoulder called a ball-and-socket joint lets you move your arm in many directions. Your skull is made of many bones that do not move. They are held together in one solid piece by suture joints.

HOW DO BONES GROW?
Bones grow or change as long as you live. Your head and other parts of your skeleton had a lot of cartilage when you were born. Bones replaced the cartilage as you got older. Bones get thicker and longer as you grow taller. Bones keep growing in teenagers. Bones stop growing longer in adults. Some bones join together as you get older. Your skeleton had more than 300 bones when you were first born. An adult has 206 bones. The longest and strongest bone in adults is the thighbone, in the upper leg.
Bones are replaced a little bit at a time even after they stop growing. This replacement goes on for as long as you live. Your body needs a mineral called calcium to keep strong bones. Milk has lots of calcium. Running and other exercise also helps build strong, thick bones. Some older people have thin, weak bones. Their bones can break easily. Getting enough calcium and exercise can help keep bones from getting weak and thin.

WHAT HAPPENS TO BROKEN BONES?
Sometimes people have accidents that break bones. Maybe they fall out of a tree or down a flight of stairs. Sometimes football players or other athletes break bones when they are playing sports.A doctor has to fix a broken bone. First, an X-ray picture shows the doctor what the broken pieces of bone look like. Then, the doctor fits the broken parts of the bone back together. This is called setting the bone. Sometimes a broken bone must be put back together with wires or pins. A broken bone should not be used until it is healed. The doctor makes a hard case called a cast for an arm or leg with a broken bone. New bone starts to grow around the break. The pieces grow together and heal the broken bone.

Bank

What would happen if there were no banks? Where would you get money? Would there even be any money? The United States and Canada each have one bank that decides how much money to issue.

WHERE DOES MONEY COME FROM?
The bank in charge of the money supply is called a central bank. Before there was a central bank, lots of banks printed money. But you couldn’t always use your money if you traveled. Stores and banks in other parts of the country might not accept it. And if your bank went out of business, you were out of luck. You lost your savings.
Today, most countries have a central bank that supervises money. In the United States, this bank is the Federal Reserve. In Canada, it’s the Bank of Canada. Because of the Federal Reserve, all Americans use the same dollar bills, quarters, and other money.

WHY DO WE PUT MONEY IN BANKS?
People put money in banks for safety and for convenience. You might keep small amounts of money in a piggy bank. But what would you do with a lot of money? You could hide it. But what if
someone found your hiding place and took your money? What if you forgot where you put it? You’d have other problems, too, especially if you were a grown-up and had to pay bills. You’d have to go to the telephone company and electric company to pay your bills each month. Banks make it easy. You pay your bills from your bank account.

HOW DO BANK ACCOUNTS WORK?
When you put money in a bank, you have a bank account. The bank gives you checks, and then you can pay bills with a check. A check is a piece of paper that is a promise to pay. You sign your name on it. When you pay your telephone bill with a check, the telephone company sends the check to its own bank. The money that you owe the telephone company then goes from your bank account to the telephone company’s bank account. The two accounts can be in different banks in different states.
Banks also issue plastic debit cards. People with accounts at the bank can use these cards to pay for things and get cash from an ATM (automatic teller machine). When your mom or dad shops, they can give the store their debit card. The store’s bank uses information on the bank card to collect the money from your mom’s or dad’s bank account.
A bank account used for writing checks is called a checking account. You can also have a savings account. People usually put money in a savings account when they don’t plan to use the money right way. The bank pays you a small amount for keeping money in a savings account. This money is called interest. Most checking accounts pay no interest.
If a bank is paying 5 percent interest, then you will get 5 cents every year for every dollar in your savings account. That doesn’t seem like a lot of money, but over time it adds up.

WHAT DO BANKS DO WITH MONEY?
Banks lend money to people who need it. Let’s say a woman wants to buy a car. However, she doesn’t have enough money right now to pay for the car. She might go to a bank and ask for a loan. The loan lets her buy the car now and slowly pay the money back to the bank. Before lending her the money, the bank will make sure the woman is able to pay back the loan. It will ask her where she works and how much money she makes. It will charge a small fee, or interest,
on the money it lends.
Today, bank accounts are insured. They weren’t always. Sometimes there were “runs” on banks when a large number of people demanded their money. This action panicked others, who also took their money out of the bank. A run on the bank began, and the bank went broke. People who didn’t get to the bank in time lost their money.
Governments in many countries decided to do something to stop runs on banks. They insured bank accounts so that people would not have to worry about losing their money. If a bank in the United States goes out of business, the insurance repays people the amount in their accounts up to a limit of $100,000 per person.