The water near the surface and warmed by the Sun is less dense than water near the bottom, because density of water changes as the temperature of the water changes. The lower the temperature of water, the higher the density of that water—until around 4ºC. At 4°C, water reaches its maximum density. Water below 4°C is lighter than water at 4°C because the water molecules are beginning to form into ice crystals. During the phase change from liquid into ice, water molecules begin to take on a crystalline structure which is less dense than liquid water. This crystalline structure (ice) floats to the surface.