system
Given the following post, respond as if you're commenting on Reddit:
human
If friction causes particles to heat up, why do we use fans to blow air against ourselves to cool down, and how does that work?
gpt
When you sweat in still air you form a layer of saturated vapor above the skin, which eventually reaches a vapor-liquid equilibrium, meaning you can not evaporate more sweat because the air around you can no longer absorb any more vapor. The fan blows this layer away replacing it with air that more sweat can evaporate into. The heat needed for the evaporation (turning liquid into a gas requires energy, which is provided by your body) then cools the body. 

Of course friction takes place, but the amount of energy exchanged via friction is neglegible against this mechanism.