"Mom, my tummy hurts!" "Dad, I think I'm going to throw up!"

How many times do think your mother and father have heard you say things like that since you were born? Even if you're really healthy, chances are you have complained about tummy upsets at some time or other.

Doctors say a stomachache is one of the most common complaints kids have. Your stomach and the rest of your digestive system have a lot to do. Your stomach is a muscular, pear-shaped bag located right between your ribs above your bellybutton.

In your stomach, the food you eat is bathed in strong substances called acids and enzymes, which dissolve your food. The stomach's muscular walls churn the juices with the food, forming a creamy liquid that moves into your small intestine, a twisting and turning tube that isn't actually small at all. Stretched out, it would be several times taller than you are.

Most of the nutrients from your food get into your body through the walls of the small intestine, but not all of the food you eat can be used. Some of it passes into the large intestine -- a coiled tube about five feet long; it is called the large intestine because it has a larger opening than the small intestine does.

The walls of the large intestine soak up water from your food and return it to the body. The solid, useless parts of your food pass out of your large intestine when you go to the bathroom.

Some parts of the digestion process are fast. You chew your food for only a few seconds, and it moves on to the stomach in a few seconds more. The stomach churns your meal for several hours before it turns it over to your small intestine, where it may take as long as eight hours getting through. A meal takes between 12 and 15 hours to go through the whole digestive system.

Things can go wrong along the way, leading to stomachaches, but most aren't that serious, doctors say. One common cause of stomachaches in kids is constipation. This happens when you don't eat enough bulky food or drink enough water to keep your digestive system working efficiently and getting rid of waste products easily.

Some kids get stomachaches when they get mad, worried or stressed out. Doctors call these kids "gut reactors." When they're upset, they may feel queasy or throw up or get stomach cramps. Doctors say that these kids produce extra acid in their stomachs when they're feeling emotional.

Some kids gulp air when they're upset, and that can cause a stomachache, too. Some get stomachaches because their bodies have trouble digesting dairy products. Milk contains lactose, a form of sugar. To digest lactose, the body makes an enzyme called lactase. Not everyone makes enough lactase, and if you're one of those people, you get a swollen stomach, gas and pain when you eat ice cream, cheese or yogurt or drink milk.

If people who are lactose-intolerant avoid foods containing milk, their stomachs feel fine. Of course, just overeating can make your stomach hurt. To keep your digestive system working smoothly, it's important to pay attention to that signal that says, "I've had enough." If you stuff too much pizza or popcorn or ice cream into your body, it's going to let you know how it feels about it. And it doesn't feel good.

Not eating on schedule can cause problems, too. How do you know? Your stomach growls at you. You've probably noticed that this often seems to happen during quiet times like study hall. When your stomach growls with hunger, it's a sign that it's doing its job: creating digestive juices and churning them around inside. The muscular stomach walls whoosh the juices around like a washing machine churning water, and the sounds begin as a cue it is time to eat -- but not overeat.

Some stomachaches are a sign of illness or even an emergency. Flu can make your stomach --

and the rest of you -- feel really awful. If you ever get stomach flu, drink plenty of fluids to keep your fever down and your body from getting dehydrated, or dried out.

Food poisoning, which is caused by eating food that contains germs, is another stomach problem that can feel a lot like flu. It causes vomiting, diarrhea and a high fever. When someone has food poisoning, it's important to drink enough fluids.

Then there's appendicitis. Appendicitis is an illness that happens when a small, extra part of your intestine swells up. The pain this infection causes usually starts in the area of the bellybutton and then spreads down and to the right side. The patient also feels feverish and queasy. If you ever feel this way, tell someone immediately. Appendicitis is a medical emergency, not just another stomachache. Appendicitis is unusual, though. It's more likely that your stomach hurts because you ate too much or ate the wrong thing. Your mom is right. You'll feel better soon.Tips for Parents

These are the danger signs of appendicitis:

Recurring spasms of pain, at first near the navel, but sometimes in the lower right side of the abdomen.

After a few hours, constant, severe aching in the lower right abdomen. A child may walk hunched over.

Tenderness in the abdomen. Pain that interferes with sleep or becomes more severe if the sufferer moves.

Nausea and often vomiting.

Refusal of food and water.

Fever, up to 102 in adults and higher in children. According to "What to Do in an Emergency" (Reader's Digest), appendicitis symptoms vary and are hard to diagnose. If you suspect that your child's stomachache could be appendicitis, call a physician fast.

Catherine O'Neill is a freelance children's writer.