Your body works best when its temperature is about 98.6 degrees Fahrenheit (37 degrees Celsius). 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: hy-poh-thah-luh-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: per-spuh-ray-shun), and it is made almost completely of water, with tiny amounts of other chemicals like ammonia (say: uh-mow-nyuh), urea (say: you-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-dray-ted).

- Answer provided by KidsHealth.org