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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.

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