When you tickle a baby's tummy, you're usually rewarded with a grin and a giggle. Have you ever wondered why? What's so funny about tickling, anyway?

If you're ticklish, you may get teased by your friends sometimes. If someone walks up to you wiggling his fingers, you may start laughing before he even touches you. Then when you're actually being tickled, you scream with laughter, and fall down on the floor. "Boy, are you sensitive!" your friend might say.

Your friend is right. You are sensitive -- and it's your sense of touch that's at work as you wriggle and laugh when being tickled.

Human beings have five senses -- hearing, sight, smell, taste and touch. Scientists know a lot about how these senses work, but they don't know everything. There are still some mysteries to be cleared up about the sense of touch, for example. One of these is how the "tickle" message gets from your friend's wiggly fingers to your brain.

Each sense has an organ it uses to get messages to your brain. To hear, you use your ears. To see, you use your eyes. Your nose and mouth send smell and taste messages to your brain. And you sense touch with your skin.

In your skin, or just beneath it, there are small bundles of cells called receptors. They're called receptors because they receive messages. Your body contains an enormous network of different kinds of receptors. Some of them can sense temperature. Others sense pressure. Others sense pain.

Let's say you've just woken up in the morning. Your head is cradled by a feather pillow, and as you turn over, one of the feathers pops out of a hole in the seam and lands on your face. A receptor in your skin sends a signal to your brain about the feather. The message? "Soft."

Next, your mother calls to you to get moving or you'll be late for school. You peer out the window and see the gray winter day outside. You pull the blankets up over your shoulders again and snuggle down. All over your skin, receptors pick up a message. It says "warm." But you have to go to school, so you climb out of bed, and put your bare feet on the floor. Two more messages flash to your brain. They say "cold" and "hard." Then as you shuffle off to the bathroom, you get a splinter in your foot. The message this time? "Pain."

At the breakfast table, your brother leans over and tickles you under the arm. "Smile," he says. You can't help it. You're ticklish, and you laugh.

The receptor that sent that "tickle" message to your brain is a special kind. The nerve endings that sense tickles and itches are called "free" nerve endings. You have lots of free nerve endings in your body, and researchers are still at work trying to figure out exactly what part they play in your sense of touch. (Some of the other kinds of receptors you have are named after the scientists who discovered them. Certain pressure receptors are called Ruffini's Endings and Merkel's Disks. Maybe if you become a scientist and study tickling, there will be a receptor named after you someday.)

Some touch receptors are extremely sensitive. They can feel the movement of the fine hairs on your forearm, for example. They can sense a touch so brief that it lasts only a tenth of a second, or they can feel longer, stronger contact like a big bear hug. Sensitive pain receptors send signals so fast that they make you pull your hand away from a hot stove even before you know you've touched it.

Scientists used to think that the "tickle" reaction was caused by mild activity around pain receptors. But recent studies show that the ticklish feeling comes from nerve endings in the very outer layers of your skin. When these nerve endings are irritated, they cause that familiar giggling and wiggling reaction -- in some people. Other people aren't ticklish at all.

To reach the brain, touch signals travel along a network of nerves. The signals travel in the form of tiny bursts of electricity. A special section of your brain translates them into information like "tickle" or "pat" or "smooth" or "icy." This is a terribly important part of your brain. After all, your sense of touch affects your safety by keeping you away from things that hurt. It allows you to experience nice feelings like the glow of a warm fire, or the softness of a kitten's fur.

Even more important, your sense of touch allows you to experience human contact -- like hugging. If we are touched and cuddled a lot as babies and children, it helps us become trusting, confident, affectionate people when we grow up.

So next time you're around a baby or a small child, give him or her a gentle tickle, and then a hug. That way, you'll give several kinds of touch receptors a workout. Tips for Parents

By now the vital function of touch is part of the conventional wisdom of parenting toddlers and infants. But what about your school-age child? Research conducted at the University of California at San Francisco suggests that physical affection continues to be a factor in healthy development. A study conducted among 8- to 10-year-olds by Sandra Weiss, associate dean of the UC-San Francisco School of Nursing, showed a correlation between physical affection from both mothers and fathers and the development of a positive body image. Weiss recommends playful rough-housing and frequent, easy affection as ways to make your kids feel lovable during this challenging developmental stage.