What you lose besides water when you sweat

Sweat tastes salty, and that is the clue. When the eccrine gland, which is the sweat gland whose job is to cool you down, starts producing fluid, that fluid is not pure water: it carries sodium and potassium ions, which are the body's salts.

But between the gland and the skin there is a stretch of tubing, the duct, and something interesting happens there: the wall of the duct takes back most of those salts before they get out. It is a filter that takes back the salt and lets the water through. That is why what ends up wetting the skin holds a much higher proportion of water, though it keeps enough salt for you to notice it.

The gland switches on when body temperature rises, and it makes no difference whether it rises from exercise, from outside heat, or from fever. And evaporating that sweat carries off about 22% of all the heat the body loses.

And chloride, another of the ions in sweat, does not come out in a fixed amount either: how much of it comes out depends on how fast you are sweating.