On a warm day, you run in from the playground and head for the water fountain yelling, "I'm thirsty!" At home, you munch on a salty snack, say "I'm thirsty," and walk over to the refrigerator. In bed, you wake up in the middle of the night and mumble, "I'm thirsty," hoping your mom or dad will stumble in to bring you a drink.

Then there are other times when you think you can't possibly drink another drop. After you have gulped half a bottle of juice, or drunk a tall glass of water to wash down a pill, you may not like the idea of pouring more liquid into your body.

Why are you sometimes thirsty and sometimes not?

It's your body's way of making sure that your cells -- the tiny parts that form your tissues, organs and blood -- contain just exactly the right amount of water to work well. The part of your body you can't see is called your internal environment. It's pretty damp. In fact, more than half of it is water. To be healthy, your internal environment has to be just right -- not too damp, not too dry. So your body comes equipped with a system that keeps track of the level of water inside.

When you think about water inside your body, you may imagine that you're like a big bottle half-filled with liquid. It's not really like that at all. Much of your weight comes from water -- but it's not sloshing around. It's spread throughout the billions of cells of your body. Some of the water is inside your cells. The rest of it bathes the outside of your cells, creating a healthy enivronment for them.

The amount of water in your cells changes all the time. As you breathe, you send some water vapor out of your body. When you sweat, you get rid of more water. Over the course of a day, you lose nearly two quarts of water when you go to the bathroom. All that lost fluid has to be replaced -- so you drink.

The feeling of thirst is your body's way of letting you know when it's time to drink something. Inside you, your brain and nerves work together to keep track of the amount of water in and around cells. When the level gets too low, you experience that dry feeling in the back of your throat that makes you want to drink something.

One way that nerves help keep track of the water in your cells is by measuring the amount of salt mixed with it. All human beings must have some salt inside them to stay alive. The salt and water mixture in your cells must stay at a precise level in order to keep your body working. If the balance of salt and other chemicals in your internal water supply is not right, you may get sick.

One of salt's jobs is to help your blood keep circulating -- which is a very important job. When the level of water in your body drops, the salt level goes up. The nerve cells sense this, and sound the "Time for a drink" alarm. You drink, and the water mixes in with the salt to make a weaker -- and healthier -- bath for your cells. That's why eating salty food makes you thirsty -- your body has to signal you to drink more and keep the salt level from going too high.

When nerves in your brain sense that there's too much water in and around your cells, they start a different process. This one involves producing chemical messengers called hormones. These subtances speed off to two bean-shaped organs called kidneys. The kidneys are in charge of regulating the flow of water out of your body in the form of urine. The hormones stimulate the kidneys to produce more urine, which washes the extra water out of your body when you go to the bathroom. You can read more about how the kidneys do this job on next week's How & Why page.

Without water, you would last only a few days. So drink up. Like the oxygen in the air you breathe, water is essential for life. You get some of the water you need from food -- especially fruits and vegetables -- and from liquids like milk, juice or soda. Even so, drinking plenty of plain water every day is good idea. It bathes your cells, carries nutrients to the places in your body that need them, helps carry wastes away, and keeps your body temperature at a healthy level. Tips for Parents

"For a healthy, functioning body you need to provide the kidneys with enough water to dilute waste products and eliminate them from the body," says Dr. Glenn Bock, a nephrologist at Children's Hospital National Medical Center in Washington. "Thirst is one of the profoundest drives in the human being, and children tend to be smart about responding to it. Even so, parents often call me to say, 'My kids drink so much, I'm worried about it.' "

In those cases, Bock says, there is rarely a problem. "Unless your children are drinking so much liquid that it interferes with normal food intake, I'd say let them have relatively free access to healthy fluids -- juices, clear water, clear soups, things like that." What's not desirable, Bock adds, is depending on sugary drinks loaded with caffeine or other additives to slake thirst. Kids do ingest water with such drinks -- but they also get a lot of things they don't need at the same time.