What do you, the trees, and a hamster have in common? Give up? You all need water. All living things must have water to survive, whether they get it from a water fountain, a rain cloud, or a little bottle attached to the side of a hamster cage.

It might not seem like it, but water is the most necessary nutrient of them all - so necessary that people can't survive for more than a few days without it. More than half of the weight of your body is water - if you weigh 60 pounds (27 kilograms), fewer than 25 of those pounds (11 kilograms) are minerals and other solid stuff. The remaining pounds are water.

So let's see what water does for our bodies . . . jump in and get wet!

You're So Waterful
Just like peanuts are the main ingredient in peanut butter, water is the main ingredient in the fluids in your body systems. Fluids travel through your body, carrying nutrients and waste to and from all your cells and organs. Your heart, your eyes, your intestines, and even your big toe need water-based fluids to survive.

So what are some of the watery fluids that flow through you day and night? In your circulatory system, water is the main ingredient in blood. Water is also in lymph (say: limf), a fluid that is carried by your lymphatic (say: lim-fah-tik) system, which is sort of like a second circulatory system.

Water is also the basis of the juices in your digestive system, so you can digest food properly. And it's the main ingredient in urine (pee), helping you get rid of liquid waste. Don't forget perspiration (also called sweat): This is water that comes through the skin, as part of your body's temperature-regulating system.

Water Keeps Things Lubricated
If you've ever heard a squeaky door or wheel, you know the sound of something that needs to be lubricated (say: loo-bruh-kayt-ed). Lubricating means making it easier for something to move by keeping the parts from rubbing together. In the case of a squeaky object, people usually put a little oil on the rubbing parts so they move more easily. But in the case of your body, it isn't anything in a hardware store that does the job: It's water!

Water is in charge when it comes to keeping things moving freely because it's a big part of the fluid that lubricates the body's joints. Joints are the places where bones meet, like your knees or your elbows. Try straightening out your leg or bending your fingers - it's easy when you have water helping out!

Water is also the biggest part of mucus (say: myoo-kus), the slimy substance that's in your eyes, nose, throat, and many other parts of your body that you can't see, like your stomach. Sometimes when you have a cold and your nose is running everywhere, you might wish you never even heard of mucus. But the truth is, you need it to keep things lubricated and running right in your body.

The same goes for saliva (say: suh-lye-vuh), which is also known as spit. It is made mostly of water, and it keep things lubricated in your mouth and down into your digestive system.

Water Makes Things Move
As you digest food, the food moves through your intestines. At the end of the trip, the stuff your body doesn't need gets ready to leave your body as waste, which is called a bowel movement. And if you guessed that water is the ingredient that moves everything along, you're right! Imagine you have a tube stuffed with pieces of bread and you're trying to use your fingers to push the bread through to the other end of the tube.

Now imagine that same tube, with those same pieces of bread - only this time the bread is a little wet. Do you think it would be easier to push the bread to the end, now that there's water there? Water helps your intestines do their job more efficiently by keeping things moist.

Now what if you had more and more bread, but hardly any water in the tube? Well, if you've ever had constipation, that's sort of what this is like. Constipation means you're not moving your bowels enough, and your bowel movements are hard and dry - they can't get through the tube in your body that's your large intestine. That's why water can help prevent constipation, by adding more liquid and keeping things moving on through.

Water Keeps Things Cool
People's bodies like to be at about 98.6 degrees Fahrenheit (37 degrees Celsius). But if you're out running around or exercising in warm weather, you can feel pretty hot. And sometimes you don't even need to be exercising - if the weather's warm enough, you can feel hot just sitting still.

And that's where water comes in, by cooling you down with sweat. When your body generates a lot of heat, water comes up through your skin as sweat, and it evaporates into the air. (Evaporation happens when a liquid changes into a vapor - like when water boils and turns into steam.)

As the sweat evaporates, it cools down your skin, which cools down your blood. When your blood is cooler, your insides become cooler, and your whole body cools down. It's like your body's own personal air conditioner!

Replace That Water!
As you can see, water works hard in your body every day. That's why it's important to give your body the water it needs to carry out all its different jobs. On a regular day, your body loses 2 to 3 quarts of water - enough to fill a 2-liter soda bottle. You probably guessed that water is lost when you pee, have bowel movements, and sweat - especially on a hot day.

But your body also loses water in an almost invisible way - evaporation. Water escapes from your body through your skin and in your breath. How? Your breath contains droplets of water that are usually too tiny to see. They become visible for a quick second in the cold when you can "see your breath." But water evaporates this way all year round.

Your skin also contains water, which escapes in tiny droplets that go into the air. If you set a wet sponge on the kitchen counter, after a while, the sponge will dry out as the water evaporates. Likewise, the water in your skin evaporates into the air, but fortunately, your body can replace the water it loses so your skin doesn't dry out.

Eating and drinking is one way to replace the water your body loses. Most foods have a lot of water in them, especially fruits and vegetables. Grapes, watermelon, oranges, and cantaloupe are delicious and full of water. On the veggie side, lettuce, cucumbers, and celery also are full of water.

But you can't rely on food alone to get that water back, and that's why it's also a good idea to drink water every day. You might have heard someone say that people need to drink 6 to 8 cups (1.4 to 1.9 liters) of water each day. Depending on how much water you're getting from foods and other liquids, 6 to 8 cups (1.4 to 1.9 liters) might not be necessary. If you're eating a lot of fruits and veggies, or you like to eat soup every day, you'll be getting some of the water your body needs.

But drinking water is a healthful thing to do - and although you might not need 6 to 8 cups (1.4 to 1.9 liters), water contains no calories, no matter how much you drink.

And what about using other liquids that you like to help you get that water back? Low-fat milk is a good bet, or you can mix seltzer water with juice to make a tasty, sparkling drink. Try mixing half water and half juice for a delicious water-juice combination. Soda and iced tea are OK every once in a while, but you shouldn't drink them too often because they usually come with lots of sugar and caffeine.

Caffeine is a diuretic (say: dye-yuh-reh-tik), which means it makes you urinate more. It's not helpful to be losing water when you're trying to replace it. Calories are another concern in caffeinated drinks, such as soda. Too much of almost any drink - besides water or no-calorie diet drinks - can give you too many calories.

If you're exercising and sweating, drinking water is doubly important (cold water is better). When you sweat a lot, your body loses even more water than normal. The more you exercise and the more you sweat, the more water you'll need to drink. When it's hot or humid, it's even more important to drink water, because you are at a higher risk for heat exhaustion. Here are some guidelines for the amounts of water you should drink before, during, and after exercising:

• 1 to 2 hours before exercising: 10 to 14 ounces of cold water (about a cup and a half, or 295 to 414 milliliters)
• 10 to 15 minutes before exercising: 10 ounces of cold water (about a cup and a quarter, or 295 milliliters)
• While exercising: 3 to 4 ounces of cold water every 15 minutes (about a half of a cup, or 89 to 118 milliliters)
• After exercising: 2 cups (about half a liter) of cold water for every pound of weight loss through sweat and evaporation (this means about a cup or 2 for most kids; if it's a hot day you may feel thirsty enough to drink even more)

When you're exercising, don't wait until you're thirsty to drink up. Did you ever get a very dry mouth and suddenly feel very thirsty? That's because your body already needed water and was trying to tell you by keeping water from going to the salivary glands (these glands make saliva, or spit, which is made from water, remember?). So your mouth felt all dried out, and you had a drink.

But when it's hot or you're exercising a lot, it's much better for you to drink even when you're not thirsty. You'll feel better and have more energy.

Water is great - it helps every cell in our bodies, keeps things moving the way they should, and even cools us down when we need it! Keep your body healthy by drinking enough liquids all the time and especially when you're exercising. With water working for you, you're sure to feel H₂0-so-cool!

Updated and reviewed by: Ellen Kempf, MD
Date reviewed: August 2004
Originally reviewed by: Heidi Kecskemethy, RD, CSP