Question

How does perspiration or sweating help keep our bodies cool on a hot day​

Answer 1

When the sweat evaporates, it takes the latent heat of vaporisation from our body hence making our body cool.

Answer 2

When the sweat hits the air, the air makes it evaporate (this means it turns from a liquid to a vapor). As the sweat evaporates off your skin, you cool down. Sweat is a great cooling system, but if you're sweating a lot on a hot day or after playing hard you could be losing too much water through your skin.