People have said that it brings up warmer hot chocolate from the bottom of the mug, but why isn’t the entirety of the hot chocolate the same temperature?
In: Physics
Heat travels from hotter to cooler objects. If you want hot chocolate to cool down you need to expose it to something that is cooler than it.
The only such portion of liquid hot chocolate in a cup is the one that is at the top, because it is in direct contact with the cooler air. So the top layer cools down fast. The other layers that are underneath can then slowly transfer their heat to the layer at the top, which eventually again lends it to the cooler air. That happens when you don’t stir your chocolate cup at all. Eventually it will cool down on its own because of this heat transfer.
However, you can hasten the cooling by sending the top layer to the bottom and bringing up the bottom layers. This is what stirring does. Now the hotter layers which were earlier buried underneath, come up and in contact with the air and lose heat. As you keep stirring, the hotter layers keep coming up, releasing the heat, in turn, cooling your chocolate pot fast.
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