Dehydration happens when your body loses more water than you're taking in. Your body is mostly water: about 60 percent of you is H₂O. You need water for everything from digesting food to thinking clearly to keeping your temperature steady. When you don't have enough, you become dehydrated.

You lose water constantly through sweating, breathing, and using the bathroom. Usually you replace it by drinking and eating (many foods contain lots of water). But if you're playing hard outside on a hot day, or if you're sick with a fever or stomach bug, you might lose water faster than you're replacing it.

Early signs of dehydration include feeling thirsty, tired, or dizzy, or noticing that your pee is dark yellow instead of pale yellow. Severe dehydration is dangerous and requires medical attention. Athletes, hikers, and people working outdoors in heat need to drink water regularly to avoid dehydration.

Scientists use dehydrate as a verb to describe removing water from something on purpose, like making dehydrated camping food that becomes lightweight and shelf-stable.