Water coolso object by heat absorption Why do water of water based liquids like sweat cool objects? The most clear example I have is the sweat on the skin. I learned that it absorbed heat because it evaporates, but this is something strange. I also read that by putting a little water on a can under the sun will cool the inside of the can, for the same reason as sweat. Why is this?
Why does a liquid take away the heat, while it isn't even at the boiling point? The body is only 37 degrees Celcius, why does sweat "want" to evaporate already? I do know that liquids take energy to change its phase without adding to the temperature, but this is only at boiling point.
 A: Well you almost got it but then talked yourself out of it.  Yes it does take energy (latent heat) to convert a liquid (water) to a vapor .  That is the energy to overcome thing like Van der Waals forces that hold close packed molecules together and allow them to float free of such constraints.
But it is NOT true, that this only happens at the Boiling point.   It happens at any Temperature.   It takes approximately 593 calories per gram of water (at 100 deg C) to convert it into steam at 100 deg C.
At a lower Temperature, the water molecules will have less average kinetic energy, so it will take more energy to convert it to vapor at the lower Temperature, so the latent heat of evaporation will be somewhat higher than 593 calories per gram.
As a rough figure, you can assume one more calorie per gram for each degree lower Temperature.  So for ice water at near zero deg C it will take about 693 Calories per gram.
If the water is on your skin, that latent heat will be extracted from your warmer skin, which is why it makes you feel cooler.
The Temperature of a mass of material, reflects the average kinetic energy per molecule of the material.  But individual molecules have diffent energies which they exchange during their collisions.  Thes energies generally are distributed with some statistical distribution, often a Maxwell-Boltzmann distribution, which has a long high energy tail to it.  So there always are molecules with sufficient energy to escape the surface of the liquid, and fly free in the gaseous phase.   This lowers the average KE of the remainder , so it lowers the Temperature of the liquid.   But more high energy tail molecules will escape, anyhow, so evaporation can take place even well below the boiling point.  Of course if the atmosphere above the liquid should start to have more molecues of the liquid, (higher humidity for example), then some of those can collide with the liquid surface and be re-absorbed.   Evaporation will stop, when the number of higher energy molecules escaping, equals the number returning from the atmosphere above the liquid.
Hurricanes, which are powered by heat from warmer waters, suck a lot of latent heat of evaporation out of the ocean surface, so the hurricane leaves a cold surface water track behind it.
