Wanting to heat water, we put a container of water over the flame, and not alongside it. And receives quite correct, because the air heated by the flame, it becomes easier, expelled from all sides up and wrap around our vessel.
Therefore, placing the heated body over the flame, we use the heat source efficiently.
But what if we want the opposite, to cool any body with ice? Many, out of habit, put the body on ice, is set, for example, a pitcher of milk over ice. This is impractical because the air above the ice, achladies, falls down and is replaced by the surrounding warm air. Hence, the practical conclusion: if you want to cool a beverage or dish, place it not on the ice and under the ice.
Explain more. If you put a container of water on the ice, it cools only the bottom layer of the liquid, and the remainder will be surrounded by a non-refrigerated air. On the contrary, if you put a piece of ice on top of the lid of the vessel, the cooling of its contents will go faster. Chilled upper layers of the liquid will fall, with warm fluid rising from the bottom, until you are cooled by the liquid in the vessel [Clean water cools not to 0°, but only up to a temperature of 4°C, at which it has the greatest density. But in practice, and is not found necessary to cool the drinks to zero.]. On the other hand, the cooled air around the ice will fall down and will surround a vessel.