# Why do only the surface of lakes freeze?

In order for water to freeze, it must reach a temperature of ${0}^{\text{o" "C}}$. This occurs at the surface of a lake, which is why it freezes. However, water, being a special molecule, expands when it freezes, rather than contracts, meaning that the ice on top of the lake is less dense than the water underneath it. This allows it to float on top of the water.
Since the water below the ice on top of the lake never reaches ${0}^{\text{o" "C}}$, it never freezes.