Want to cool things, put them on or under ice?

To boil water, we put the kettle on top of the fire, not set next to the fire. So want to cool a thing b

When heating water, we put the kettle on the fire is absolutely correct, because the air heated by the flame will be lighter, rising all around the warm. So in this way we used the heat in the most beneficial way.

And when cooling things with ice, due to habit, many people put things on top, such as placing bottles of milk on ice. Doing so is not appropriate, because the air above the ice, after being cold, will sink and be replaced by hot air around . From that, we deduce a conclusion: if you want to cool food or drink, you should not put it on ice but place it under ice.

Because if you put the water pot on top, only the lowest water layer is cold, and the upper part is still covered by the air is not cold. Conversely, if you put ice on top of the pot, the water in the pot will cool very quickly, because the water layer above is cold, will sink and the hotter water will rise up instead, just like that until all the water in the pot will be cold (then, pure water does not cool to 0 degrees but only cold to 4 degrees Celsius, at this temperature the largest proportion of water). On the other hand, the cold air around the ice cubes will go down and encircle the pot.