natural convection happens because a fluid is lighter if is hot and it is heavier if is cold, so if a fluid has a hot part and a cold part, the hot part will naturally move upward and the cold part naturally moves downward. for example, if the water in a pot is hotter near the bottom because of the fire, it moves from the bottom to the surface. at the same time the water near the surface is colder so it moves to the bottom.