The transfer of heat in fluids is called convection. Convection only occurs in substances that can flow from place to place which are known as fluids. Fluids are water and gases. Convection starts when molecules in a fluid begin to vibrate. The molecules in liquids and gases are more spread apart than in solids. As the molecules vibrate, they move farther apart from one another. The hotter fluid is lighter in density and rises. The cold fluid moves to fill the space at the bottom left by the rising hot fluid. The heated fluid cools as it rises and sinks back down to the bottom. The fluid heats, rises, cools, and falls in a circular motion. Over time, the entire fluid is heated to an even temperature.
