Most substances shrink and get denser as they get colder. Water is odd in that it gets denser down to about 39.2 degrees, then gets less dense as the temperature drops further.

At the freezing point, there's a change in the atomic forces that hold water molecules together, and they move apart. Since they are less dense, the colder molecules float on top.

The best analogy I've seen compares water molecules to a platoon of soldiers. During training in the field, they are spread out many yards apart (water in a vapor state). Waiting to go into the mess hall, they pack close together (liquid water). On parade, they are lined up in rows one arm's-length apart (ice).

Interestingly, scientists who looked for salmon in the Great Lakes during the winter were amazed to find the fish as deep as 700 feet until they realized that the densest and warmest water would be at the bottom of the lakes. (Quote)

h2o