Since the molecules in a gas or liquid are too far apart for the heat to be effectively transferred by conduction, the heat is transferred though fluids by the motion of the gas or liquid by convection. Density of a fluid is less when it is warm than when cold, which cause density differences which cause the warmer fluid to rise, therefore setting up convection currents. For example, heating a pot of water is caused by convection. The heat is conducted through the metal which causes the warm water to rise to the top because its density is less than the surrounding cold water.
Convection currents are important in many ways, one which is to the operation of some types of heating systems.