Sara, who is a pretty funny person, recently visited a friend who was recovering from an illness. Sara thought her friend seemed kind of depressed, so she started telling a few jokes. Her friend started laughing and holding her stomach. "Don't make me laugh," she said. "It hurts."

Sara's friend might have felt some discomfort when she laughed -- but it's possible that the chuckles and giggles were doing her good. You've probably noticed that a good laugh makes people feel happier when they're sad. Some medical researchers believe that laughter can actually help people heal physically, too. It looks like the old expression "Laughter is good medicine" may be true.

Last year, nearly 1,000 people went to Boston for a two-day meeting. They went there to talk about how love and laughter help in the healing process. Some of the experts at the meeting were comedians who make their living by keeping people in stitches. Some were nurses and doctors. Others were psychologists and psychiatrists who study people's feelings and help them learn to live happier lives.

There were several people at the Boston meeting wearing buttons that read, "Warning: Laughter may be hazardous to your illness!" They are members of a group called Nurses for Laughter. The nurses in this group believe that humor can help people deal with the feelings of worry, fear and sadness that often come with illness. These nurses know that laughing alone can't cure a sick person -- but it can help make a patient feel better inside.

What actually happens in your body when you laugh? Writer Norman Cousins, who believes he cured himself of a serious disease by laughing and concentrating on positive feelings, calls laughing "internal jogging."

When you laugh, your muscles go into action, as they do when you exercise. After a good laugh, you may be more relaxed and feel like you have more energy. You feel the same way after you run around the block, or play a fast inning of softball.

Physically, laughter is pretty complicated. First, something strikes you as funny. It could be a movie, or a joke, or an expression on your friend's face. You may chuckle at a memory, or at your kitten's antics with a toy. But no matter what tickles your funny bone, you start to laugh. Your shoulders shake. Your mouth blows air out in short puffs -- which can rush out at 70 mph if you're laughing really hard. Your abdominal muscles tighten and relax. Your heart beats faster, and your rate of breathing increases. Sometimes glands in your eyes start producing tears. Your vocal cords tense up so that you can hardly speak. If you're laughing really hard you may even get a "weak in the knees" feeling as the muscles that hold your skeleton in place relax.

All that chuckling also has an effect on a part of your body called the endocrine system. This system is made up of glands. The glands produce chemical messengers called hormones that enter your bloodstream and cause things to happen. It hasn't been proved yet, but some studies suggest that laughing actually relieves pain the way taking a dose of medicine might. Some researchers think laughter may cause your glands to release chemicals that actually kill pain.

One specialist in the healing power of laughter estimates that Americans laugh about 15 times a day. Next time someone asks you if you've taken your required daily amount of vitamins to stay healthy, you can answer, "Have you had your daily laugh requirements?" That might be keeping you well, too. Tips for Parents

It may be the season to be jolly, but this holiday month can be stressful, too. Try sharing a laugh with your kids by reading one of these humorous titles recommended by the staff of the "Cheshire Cat Book Store," a children's shop at 5512 Connecticut Ave. NW; (244-3956).

*"The BFG" by Ronald Dahl (Puffin; $3.95).

*"The Snark Out Boys and the Avocado of Death" by Daniel Pinkwater (NAL; $2.25).

*"The Best Christmas Pageant Ever" by Barbara Robinson (Avon; $2.50).

*"Jacob Two-Two Meets the Hooded Fang" by Mordecai Richler (Bantam; $2.95).

*"Bunnicula" by Deborah and James Howe (Avon; $2.50).

*"Konrad" by Christine Nostlinger (Avon; $2.50).