Water behaves like other substances as it goes through temperature changes. As it loses heat (cools down), the individual water molecules have less and less energy; therefore, they move around and vibrate across smaller distances. This means that as water is cooled, its overall density increases. That's true of other substances too. But water does something weird, when it freezes. When most substances freeze (change from liquid to solid), their molecules continue to contract and increase overall density. That's why a block of steel sinks in liquid steel, or why rocks sink in lava. Water doesn't do that though. An ice cube has a lower overall density than the surrounding liquid water, which is why ice floats. When water freezes, the molecules shape up in a nice, ordered crystal structure. That organization is more "open" than the liquid water; therefore, water expands as it freezes. It expands by approximately 9% and becomes less dense.