While it feels gross, sweating is an effective way of cooling down the body. It utilizes the difference in thermal energy contained in a liquid versus that contained in a gas. A gas contains more thermal energy (because the gas molecules move fast) than a liquid. Therefore turning a liquid into a gas requires an input of energy.
When it's hot, our body wants to expend some thermal energy so it can cool to a comfortable temperature. When sweating, our body takes some of our water and puts it out on the skin, where it comes in contact with air. In air, the sweat evaporates. The water turns into water vapor, from its liquid form to its gas form. This requires an input of thermal energy, which our body is happy to provide. The water vapor floats away, carrying that extra energy with it and helping us keep cool.
To replenish the sweat, we need water and that is why when it's hot, we feel thirsty and have to drink a lot of water.
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