The reason why the surface of a lake freezes in severe winter, but the water at its bottom is still liquid is due to the unique property of water. Water attains its maximum density at 4C. When a lake cools, the surface water loses heat, becomes denser and sinks. This process continues until all the water in the lake is at 4C. With further cooling, a stable, lighter layer of water forms at the surface. When this surface water freezes, it floats and stays at the surface of the lake until a solid sheet of ice is formed. The water at the bottom remains liquid because it is denser and hence, does not rise to the surface to freeze.