Why does ice float instead of sinking in water?

Ice floats because it is less dense than liquid water, which is unusual among substances. When water freezes, its molecules arrange themselves into a hexagonal crystalline lattice held together by hydrogen bonds. This structure spaces the molecules farther apart than they are in liquid water, making ice about 9% less dense. Since density determines whether something floats or sinks, the less dense ice sits on top of the denser liquid water. This property is critical for life on Earth. Because ice floats, lakes and ponds freeze from the top down, leaving liquid water beneath where aquatic life can survive winter. If ice sank, bodies of water could freeze solid from the bottom up, making survival of aquatic organisms far more difficult.