Ice fishermen sit on top of frozen lakes in the winter and catch fish in the liquid water below through holes cut in the ice sheet. The fact that the lake doesn't freeze solid is a good thing for those fish who live there and for the hungry fishermen. In this problem we will investigate how long it takes for this sheet of ice to form and why the lake doesn't freeze solid during the winter. The water on the surface of the lake only starts to freeze when the air temperature is below the freezing temperature of water.

Why does the ice only form on the surface and not throughout the entire volume of the lake? Give your answer as a written explanation in complete sentences.