Water is the only substance (at least that I've heard of) that doesn't follow the ordinary rule of expansion with heat and contraction with cold. It contracts with cold down to freezing, and then EXPANDS! So it floats.

And a durn good thing, too! If it were not so, ice in the seas and lakes and ponds would sink, and would progressively build up, not thawing, being added to, until eventually essentially all the free water in the world would be in the form of ice, and life (at least as we think of it) would be impossible anywhere in the world.

Dave Oesterreich