system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 Why does our body sweat out salt when we need it?

Since sodium is so important to keep electrolytes up in our body why does our body sweat it out instead of just simply water?
gpt
I think we're thinking a little circularly here! We only "need" extra salts *because* we are sweating them out. We are told to keep our electrolytes up because we sweat.

The reason why we sweat is to cool down the body. Since it takes energy to turn water into a vapor, biology has evolved to use "evaporative cooling" as a way to stop our bodies from overheating. When we are hot, the heat from our bodies can be put into the water in our sweat, which will cause it to evaporate and leave our bodies.

Have you ever heard the idea that salt draws water, e.g. in cooking? We're using the same principle to get water out of our bodies, so it can wick away the heat from our bodies. Sweat contains salts because it is the best way to get the water out of our bodies. This is due to a property called "osmotic pressure", which basically says water moves into solutions that contain more solutes, like salt or sugar.