Convection

Convection is one of the three methods of heat transfer. Unlike radiation or conduction, convection only works if the heat is moving through a fluid, like water or air. As the fluid is heated, the heated fluid moves away from the source of heat, pushing colder fluid toward the heat source. As that fluid warms, the previously heated fluid cools, and gets pushed back toward the heat source.

This is what is happening when you heat water for a nice cup of cocoa. Convection evenly heats all the water you need for your cocoa using physics!

http://www.goodhousekeeping.com/food-recipes/a9478/hot-cocoa-mix-recipes/

If convection didn't exist, only the water touching the heat source would get warm and it would take much longer to heat the water. The water would also be heated unevenly so the water at the surface would be cooler than the water at the bottom, which would be extremely hot from being right next to the heat source for so long.