Merry Christmas! Each year, many people celebrate this day with presents, feasts, and visits with relatives. What did you do in your house on Christmas?

When I was a lot younger than I am now, I celebrated Christmas with my family. My older brother and I got up first, long before the sun. We woke up our parents, and sat on their bed to unload toys, candy and tangerines from our stockings. Then we had breakfast. Finally, we opened the presents under the tree -- including my birthday presents, since Dec. 25 is my birthday, too. Each person in my family took a turn unwrapping a package -- so it took us some time to get all the presents opened and admired.

Once all the packages were open, and the wrapping paper littering the living room had been burned in the fireplace, we'd call my grandmother in Pennsylvania. After we hung up from that call, I'd look around and wonder what to do next. Was Christmas over?

I remember one Christmas -- the one that happened the same day as my 10th birthday -- especially well. I was feeling pretty let down after all the excitement of opening presents and talking to my grandmother and cousins. The rest of the day stretched ahead of me, and it looked pretty empty. My brother had disappeared upstairs to read a book. What could I do? I began to feel pretty unhappy, even though it was Christmas and my birthday.

"Why don't you go for a walk?" my mother said.

Take a walk? That sounded much too ordinary for Christmas Day. But it was better than nothing. And my father said he'd go with me.

I remember walking all over our Washington neighborhood. It was a cold, dark day. The streets were very quiet. We peeked through our neighbors' windows and saw their trees lit up. We walked over to a hill in Rock Creek Park and crunched across the frozen grass; there wasn't any snow. We swung our arms and took deep breaths. By the time we got home, I felt like Christmas had started over again. And I walked in our house to discover it was time to eat my birthday cake.

Have you ever taken a brisk walk on a winter afternoon? Then you know that it can make you feel great. Have you ever wondered what happens inside your body as you walk? It's one of those things you don't have to think about -- it just happens. That afternoon when I was 10 I didn't think about how to put one foot in front of the other, or how to bend my knees. My body did that job for me, and walked me right out of my Christmas blues.

To walk, your brain and nerves cooperate with your muscles and bones to carry you forward. Doctors sometimes use a long, fancy word to describe this team. They call it the neuromusculoskeletal system.

Your skeleton is your body's support system, like the beams and girders that shape a building. Without it, you'd be a bag of skin and tissue, rather like a jellyfish. Your muscles attach to your bones. Your body contains more than 600 different groups of muscles. To move, you squeeze, or contract, your muscles. They pull on your bones and cause movement.

But how do you know when to squeeze your muscles? When my mother said, "Why don't you go for a walk?" I didn't have to think, "Okay, now I put one foot forward, bend the knee, swing my arm a bit, and then do the same thing on the other side." If I'd had to do that, I never would have made it from my bedroom into my parents' room to get my stocking on Christmas morning. The act of walking involves more than 100 muscles.

To signal your muscles to move, your brain does its own kind of "thinking," although you're not aware of it as it happens.

The brain doesn't use words to communicate with the muscles. It sends them electrical commands. These signals travel through your nerves. When the signals get to your muscles, they cause a shift in the chemical balance inside their cells. That change makes the muscle contract, or squeeze. And off you go.

All the squeezing and relaxing involved in movement is hard work, and your muscles need a supply of food and oxygen to give them the energy to do it.

This fuel is carried in your blood. When you exercise -- say, during a brisk walk on a winter afternoon -- your body has to keep the supply of fuel going. To accomplish this, you breathe more quickly, and take deeper breaths. That brings in more oxygen. Your heart starts beating faster, too, delivering more blood to your cells.

As your muscles work, they release energy in the form of heat -- which warms you up. That's why I got home from my Christmas walk with pink cheeks and a relaxed feeling all over.

This afternoon, think about giving some important parts of yourself -- your brain, your muscles, and your skeleton -- a Christmas present. Why not go for a walk? Tips for Parents

If you decide to take a Christmas walk with your children, you'll get maximum benefit from the exercise if you stretch gently before you start out and warm up by gradually increasing your pace until you're marching briskly along. Wear comfortable shoes and loose clothing, and slow down if you begin to feel stiffness or burning in your joints. Swing your arms, sing a carol, and enjoy Christmas, 1985.