Liquids and gases are called fluids.        Thermal convection occurs in liquids and gases.        Convection happens when fluids flow from a hotter region to a cooler region.           This flow causes convection currents.        Higher tier only:           The density of the hotter fluid is less than the density of the cooler fluid.              The movements of hot and cold fluid is due to changes in their density. This causes           convection currents.                      Air just above the land by the seaside is           heated by the hot land and rises.           It is then replaced by the cool air just          above the sea water.           This effect is called: Sea breeze.