heater is put underneath because the layers of water that are heated first, being lighter, rise upwards and thus a more effective mixing and heating of all the water is achieved. But when the water is being cooled, the process is exactly the reverse: the colder layers of water, being heavier, sink. Therefore if the cooling agent be put underneath, there will be no stirring effect and cooling will take a very long time. To cool the water more quickly, the ice should be put on top.