Colds and Flu

You start sneezing. Your throat gets sore and scratchy. You have to blow your nose a lot. You don’t feel very well. Do you have cold? Or do you have the flu? Colds and flu are illnesses caused by germs. They are both caused by germs called viruses. But they are caused by different kinds of viruses. Colds are often called common colds. Flu is short for influenza.

IS IT A COLD OR THE FLU?
Signs that you have a cold or the flu are called symptoms. The symptoms of a cold and the symptoms of the flu are slightly different. The symptoms of a cold are sore throat, cough, sneezing, and a stuffy, runny nose. Colds usually do not cause a fever. Flu symptoms are like cold symptoms, but the flu also causes chills, fever, and headaches. It makes you feel tired and achy all over.
A cold or the flu usually lasts about a week. Every once in a while they can lead to a more serious sickness, such as an ear infection or a lung infection called pneumonia.

CATCHING A COLD OR THE FLU
People once thought you could catch a cold from getting a chill in cold weather. They thought that wet feet or drafts of cold air could give you a cold. We now know that germs cause colds and flu. The germs are passed from one person to another. They travel in coughs and sneezes. More colds and cases of flu happen in cold weather because people spend more time together indoors when it’s cold outside. It iseasier for germs to spread when people are close together.

IS THERE A CURE?
There is no cure for the common cold. People take medicine to help their sore throats, coughs, and runny noses. Doctors say that resting in bed is the best way to treat a cold. There is no cure for the flu, either. Doctors can give medicine to make you feel better. Resting in bed and drinking lots of juice and water is the best way to treat the flu.

AVOIDING A COLD OR THE FLU
You can get a flu shot to help keep you from catching the flu. A flu shot, or vaccination, helps your body fight off flu germs if they attack. But it doesn’t always work. The flu virus keeps changing. When it changes, the old vaccine no longer works. Doctors have to keep making new vaccines. For this reason, you need a new flu shot every year. Sometimes the flu changes enough in a single year that you can still catch it even if you’ve been vaccinated. There is no vaccine against the common cold because more than 100 different kinds of viruses cause colds. These viruses also keep changing.

One thing you can do to protect against cold and flu germs is wash your hands before you eat anything or touch your face. Your hands may pick up the germs from door knobs or other things touched by someone with a cold. Washing your hands thoroughly kills the germs. You can also try not to spread germs when you have a cold or the flu. Use tissues when you sneeze. Cover your mouth when you cough. And wash your hands frequently to keep from spreading cold germs to others.

Sleep and Dreaming

Dogs sleep, cats sleep, and you sleep. All mammals and birds sleep. Scientists are not sure if fish, reptiles, and insects sleep. Big animals sleep less, and small animals sleep more. Elephants and giraffes sleep only 2 to 4 hours a day. Bats, opossums, and armadillos sleep 18 hours a day or more! Even kids need more sleep than grownups. A newborn baby sleeps 17 to 18 hours a day. A 10-year-old needs about 10 hours of sleep a night. Grownups need between six and nine hours of sleep a night. Some people need more sleep than others.

WHY DO WE HAVE TO SLEEP?
Scientists do not know for sure why you sleep. They do have some ideas. Safety could be one reason for sleep. People and other animals might sleep because it keeps them safe at night. It’s hard to see in the dark. Enemies could sneak up and attack animals that are wandering in the darkness. Most mammals and birds go to trees, underground dens, or nests at night. Prehistoric people went into caves or other shelters. They covered up with furs and fell asleep. You go into your home at night and snuggle up in bed. Sleep might also help your body work better. Things go wrong if you do not get enough sleep. It is hard to think and work and play unless you get plenty of sleep.

WHAT HAPPENS WHEN WE FALL ASLEEP?
Scientists have learned a lot about what happens when you lie down, close your eyes, and fall asleep. They study people in sleep labs. ometimes your eyeballs move back and forth while you’re asleep. They move fast. Scientists call this kind of sleep REM sleep. REM stands for rapid eye movement. Your body may twitch during REM sleep. Your brain is also very busy during REM sleep. It is almost as busy as when you are awake. What do you think your brain is doing? Here’s a clue: You dream during REM sleep. There is another kind of sleep. Your eyeballs do not move. Scientists call this NREM sleep. NREM stands for nonrapid eye movement. Your brain is not very busy during NREM sleep. You go back and forth between REM and NREM sleep all night long.

WHY DO WE DREAM?
Scientists have done many studies on dreams. They think your senses may have a lot to do with dreams. In dreams, you see and hear things. Dreaming is not like thinking about things. You have feelings during dreams. You may feel happy or angry. You feel fear if you have a nightmare. Your memories may have something to do with your dreams. Dreams are often like stories that stop before they are finished.

DO DREAMS MEAN ANYTHING?
People in ancient times looked for meaning in dreams. The ancient Egyptians believed dreams could tell the future. Some psychologists think that dreams show what people feel deep inside. They ask people to talk about their dreams. Some scientists think that dreams have no meaning. They think that dreams just come from nerve signals in your brain. Other scientists think that dreams are important for memory. They may help your brain sort out what to remember and what to forget.

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.

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!

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.