Hot water rises because its molecules are moving rapidly as they expand the water. Hot water is less dense, therefore, the same amount of water takes up the larger space. Hot water rises above the colder denser water. The resulting movement is called convection. The flowing displacement of a mass of cold fluid under large amounts of hotter fluid is the same phenomena seen with weather patterns where cold higher pressure air displaces and pushes up the hotter air. Hot air balloons also take advantage of this squeezing action.