Convection occurs when heat is transferred by a current within a fluid. When fluids are heated they rise. Water at the bottom of a pot that is being heated will rise to the top of the pot. The particles in the heated water move farther apart and move faster due to heat energy. When water moves to the top of the pot and is replaced by cooler water there is a circular motion of hot and cold water molecules. The circular flow of hot and cold fluid matter is known as a convection current.
Hot air rises, as it rises cool air flows into its place.