Percolation is the term used to describe the circulatory action of water or any liquid or semi liquid within a closed system that is exposed to a source of heat and because of the difference in temperature in that environment. Imagine a coffee pot. The heating coil below the pot heats the water closest to it. This water rises (because that’s what heat does). As it rises, it cools. The cool water sinks to the bottom to be heated again. The heat/rise, cool/fall action of the water creates a circulation of water which is called: percolation.