Early one morning you're playing on the jungle gym in the park. All of a sudden, there's a thump as one of your friends tumbles to the ground. As he falls, he reaches out toward the ground with one arm.

You carefully climb down and run over to see how your friend is doing. You notice that one of his arms looks funny. It's sticking out at an odd angle. It doesn't look like the other arm anymore. And you can tell that the arm really hurts.

This is one of those times when you should get help from an adult. Don't try to make your friend's arm look normal again by pulling it back into place. And don't tease him for crying from the pain. Your buddy has just broken an arm!

After a grown-up arrives, your friend will be taken to a doctor's office or to a hospital emergency room like the one at Children's Hospital in Washington.

Here's what will happen when he gets there:

He might meet Dr. Richard Reff. Dr. Reff sees lots of kids with broken bones every year.

The doctor discovers that your friend's arm is painful and swollen, and it doesn't look like the other arm. So he decides to make an X-ray to find out where the break is. With a special camera, a technician takes a picture of the arm that shows what the bone looks like inside.

"The X-ray helps us decide how to treat the break," says Dr. Reff. "If it's a very simple break, the next step is to apply a cast to the arm."

A doctor puts on a cast to protect the broken arm from getting hurt again. The doctor might have to straighten the arm first. The cast keeps the bone from moving as it heals. And because your friend can't move the broken arm, he feels less pain.

What's going on under that thick white cast as the bone heals?

Dr. Reff says that you can see what happens when a bone breaks by doing a demonstration with a wooden pencil.

"Take a pencil and snap it in two," the doctor says. "When you try to put the two ends back together, you'll notice that it's very difficult to make them fit perfectly. But if someone helps you, you can hold the two ends together while your partner puts putty or clay around the ends. When your body goes to work to repair your broken bone, it makes a substance called healing callus that works like that putty or clay."

But unlike the pencil, the broken bone doesn't end up with a lumpy bump holding it together. In time, the healing callus becomes hard and smooth, and takes the shape the bone had before it was broken.

Once the healing is complete, the broken place hardly shows -- even on an X-ray.

Your bones, although they're hard, are very much alive. Inside, they're light and spongy. In fact, your bones are about one-quarter water.

Doctors often see a kind of break in young people's bones that they call a "green stick fracture." If you try to break a green, living stick in half, you'll see how the fracture gets its name. When you try to snap a green twig in two, it bends. One side splits, but the other holds together. That's what happens in a green stick fracture of a bone, too. Children are more likely to have green stick fractures because their bones are still supple and growing, and contain a lot of water -- like a young tree!

When a bone breaks, it starts to heal itself right away. The healing callus forms in a few hours. After a couple of days, a layer of stiff tissue has formed around the break. It makes a natural cast to hold the bone in place -- in the same way that the plaster cast keeps the arm still. At the broken edges, bone cells grow toward each other and join.

This amazing repair job takes only about four to six weeks in children. Adult bones heal much more slowly. A break that puts your friend in a cast for four weeks might put an adult in one for four months.

As the broken arm heals, the doctor makes more X-ray pictures of it, right through the cast. When the healing looks complete, the doctor will hold the arm and press on the place where the break was. If there's no more pain, it's time to remove the cast.

"We use a special type of tool to do that," says Dr. Reff. "It looks like a saw, and it's very noisy. It cuts through the plaster."

When the doctor put the cast on the arm in the first place, he covered the skin with some padding. When the cast is being removed, he stops the saw when it reaches that padding. You don't need to worry that the blade will get as far as the skin.

When your friend's cast comes off, he may get to bring it home. He'll probably want to save it if it has a lot of autographs or drawings on it. But if it's an especially colorful cast, he may want to donate it to Children's Hospital, where they have a display of some of the most artistic casts that they've seen. Tips for Parents

Don't let your child's cast get wet. If the padding between the cast and the skin gets wet, says Dr. Richard Reff, "it's like wearing a wet sock for about three weeks."

Don't let your child put anything inside the cast to scratch the skin. Lack of exposure to the air and light makes the skin very sensitive, and if the child scratches the skin, it will not heal well under the cast. If the padding inside the cast gets bunched up, it can cause painful pressure sores.

Blow warm air into the cast using a hair dryer on a low setting if the skin under a cast is itching.

Use a large plastic bag to encase the cast when your child takes a bath or goes out on a wet or snowy day.