Here’s what Professor Hallux has to say:
This is all to do with the temperature of your body. To work at its best, the body likes its temperature to be around 37 degrees celsius.
If we’re exercising though or sitting on a sunny beach then the chances are that our body will start getting hotter than this. So the brain sends a message to the body telling it to sweat.
Sweat is made up mainly of water and when it is released from glands in the skin into the world it evaporates which helps cool your body down.
Naturally though this uses up water and that’s why if you’re sweating a lot it’s good to make sure you drink lots!