Sweating is one of the main ways the human body prevents overheating.
When your internal temperature starts rising, the brain can activate sweat glands in the skin.
Those glands release fluid.
The most important step happens next.
The sweat evaporates.
Sweat is mostly water
Sweat contains mostly water along with dissolved salts and small amounts of other substances.
The fluid itself does not magically pull heat out of you just because it appeared on the skin.
Cooling depends heavily on evaporation.
Why does evaporation cool you?
Changing liquid water into water vapor requires energy.
Some of that energy comes from heat at the surface of your skin.
As sweat evaporates, heat leaves with it.
Your skin cools, and blood flowing near the skin can carry some of that cooling effect back toward the rest of the body.
Why is humid weather so uncomfortable?
High humidity makes evaporation harder.
The surrounding air already contains a lot of water vapor, so sweat does not leave the skin as efficiently.
You may be covered in sweat and still have trouble cooling down.
That is why a hot humid day can feel much worse than the same temperature in dry air.
Why do we sweat during exercise?
Working muscles produce heat.
The harder they work, the more heat the body has to manage.
Sweating increases as part of the body’s effort to keep internal temperature within a safe range.
Why do people sweat when nervous?
Sweating is not controlled only by body temperature.
Stress and emotion can activate the autonomic nervous system.
That can trigger sweating, especially on the palms, soles, forehead, and underarms.
So sweaty hands before a speech do not mean the room suddenly became too hot.
Does sweating mean you are burning more calories?
Not in any useful direct way.
Sweating reflects heat regulation, not calorie burn.
A person can sweat heavily in a hot room while doing very little exercise.
Losing water through sweat can make body weight temporarily drop, but that is fluid loss, not the same thing as losing body fat.
So sweat itself is not the cooling part.
Evaporation is.
Your body basically pours water on its own radiator.
The same body systems connect with Why does sweat smell?, another everyday effect with its own underlying mechanism.
And that’s the short explanation.
Related explanations: Why does sweat smell?, Why do we blush?, and Why do we get thirsty?