It's near the end of math class. Everyone's working on a few problems. The room is quiet, except for the sound of pencils scratching across paper. You've already finished, and as you wait for your teacher to collect the paper you start thinking about the chicken sandwich on whole wheat bread that's in your lunchbox. You can imagine your teeth taking the first bite . . .

Suddenly, you hear a strange sound: GRRRRRRRR. It's coming from your stomach. How embarrassing!

Everyone's stomach growls now and then. In fact, stomach-growling has a fancy scientific name. Next time your stomach starts to growl, tell your giggling friend that it's nothing to worry about, you just have borborygmi.

Borborygmi may make you blush when it happens in a quiet place, but the noise is a normal event. It's a sign that your stomach is doing its job -- creating liquids called digestive juices, and churning them around inside. The muscular walls of your stomach whoosh the juices around like a washing machine churning water, and the sounds begin. When you feel the churnings, you know that you're hungry. After a meal, your stomach and the rest of your digestive tract continue to make noises. You can hear them if you put your ear against a friend's belly. Those gurgling sounds are the side effects of a process called digestion. Digestion is the way your body breaks food down into the separate substances it uses to fuel all your activities, whether you're humming a tune, jumping rope or reading a book.

If you play soccer outside on a cold day, you feel very hungry afterward. You may even run into the kitchen at home yelling, "I'm starving!" That's an exaggeration, of course. It takes a lot longer than the length of a soccer game for the human body to really start running out of fuel. But you are getting a powerful signal from your body that says, "Feed me."

Then, after you gobble up a snack, you feel satisfied. If someone offers you a second helping, you may say, "I'm stuffed." Even if there's a piece of your favorite kind of pie in front of you, you can't imagine eating it.

You feel full because you are full, of course. Your stomach -- a muscular, pear-shaped bag located right between your ribs above your bellybutton -- has something to work on. Nerve endings in your stomach send signals to your brain saying, "Enough."

The feeling you call hunger is pretty complicated. It's not just a matter of emptying your stomach and filling it up again at regular times. Your brain plays a role in eating patterns, too.

After your stomach has done its job and sent the food along to the next part of your digestive system, you still won't feel hungry for a while. That's because the special part of your brain that controls your appetite knows that your body still has enough fuel to keep it working for a while. It's like the gas gauge on a car -- when it gets near "empty," the driver knows that it's time to refill the tank.

The device that keeps track of how warm your house is is called a thermostat. Scientists call the part of your brain that controls eating behavior your appestat. They are still trying to figure out exactly how the appestat decides when to tell you to eat and to stop eating. They know that it is located in a tiny part of your brain called the hypothalamus. This walnut-sized section of your brain is also in charge of such vital things as your body temperature.

One experiment that helped scientists discover the hypothalamus's part in controlling appetite involved white mice. Researchers compared two kinds of mice -- a normal one, and one whose hypothalamus had been changed through surgery. The mouse with the changed hypothalamus soon grew twice as fat as the normal mouse. What had happened? The balance between how much food it ate and how much food it used up through activity was thrown off. The fat mouse no longer knew when to stop eating.

Your appestat must monitor many different chemical processes to keep you eating healthy amounts. It probably keeps track of a substance called glucose, or blood sugar. When your glucose level is low, you may get hungry. When it's high, you feel satisfied. Your brain may also sense tiny changes in body temperature after you eat, and start the signal that says, "I've had enough." Scientists would like to know a lot more about how appetite is regulated. This knowledge could help them treat diseases like obesity, which makes people dangerously fat, and other eating disorders.

Your imagination can play a role in stimulating your appestat, too. Thanksgiving is a week from tomorrow. Think about the turkey. Try to imagine what it looks like as it's lifted from the oven. Imagine the delicious vegetables and potatoes. Think about the pies, and all the other good things you'll be eating a week from tomorrow.

Feeling hungry yet? Tips for Parents

Does your child wake up hungry? Breakfast may not literally "stick to the ribs," but it does help a youngster get through the day. In "Stress in Children" (Arbor House; $14.95) psychologist Bettie B. Young outlines a diet designed to help today's over-programmed children function at their best. She suggests making breakfast a meal that includes both fruit and grains to steadily release glucose and feed the brain all morning, rather than a sugary cereal that will send your kids off feeling speeded-up, but drop them into a slump long before lunchtime.