You're biking up a hill, pedaling as hard as you can. You're almost there and — what's this? Your back is all wet and so is your face. Don't sweat it — it's only sweat!

Your body works best when its temperature is about 98.6ºF (37ºC). When your body gets hotter than that, your brain doesn't like it — it wants your body to stay cool and comfortable. So the part of your brain that controls temperature, called the hypothalamus (say: hi-po-THAL-uh-mus), sends a message to your body, telling it to sweat.

Then special glands in your skin called — what else? — sweat glands start making sweat. Sweat is also known as perspiration (say: pur-spuh-RAY-shun), and it is made almost completely of water, with tiny amounts of other chemicals like ammonia (say: uh-MOWN-yuh), urea (say: yoo-REE-uh), salts, and sugar. (Ammonia and urea are left over when your body breaks down protein.)

The sweat leaves your skin through tiny holes called pores. When the sweat hits the air, the air makes it evaporate (this means it turns from a liquid to a vapor). As the sweat evaporates off your skin, you cool down.

Sweat is a great cooling system, but if you're sweating a lot on a hot day or after playing hard you could be losing too much water through your skin. Then you need to put liquid back in your body by drinking plenty of water so you won't get dehydrated (say: dee-HI-drayt-ed).

Why Does Sweat Smell?

Sweat isn't just wet — it can be kind of stinky, too. But the next time you get a whiff of yourself after running around outside and want to blame your sweat glands, hold on!

Sweat by itself doesn't smell at all. It's the bacteria that live on your skin that mix with the sweat and give it a stinky smell. And when you reach puberty, special hormones affect the glands in your armpits — these glands make sweat that can really smell.

Luckily, regular washing with soap and water can usually keep stinky sweat under control. Many teens and adults also find that wearing deodorant (say: dee-OH-der-ent) or antiperspirant (say: an-tee-PUR-sper-ent) helps.

So don't worry about a little sweat — it's totally normal and everybody sweats. Sometimes too much sweating can be a sign that there is something wrong in the body, but this is rare in kids. If you notice more sweat, it's usually just a sign that it's time to start using a deodorant or antiperspirant. But if you think you have a sweat problem, talk to your parent or your doctor about it.