Why does ice float on water?

Because water expands as it freezes, by about nine per cent. Nearly every other substance contracts when it solidifies and its solid sinks in its own liquid. Water's molecules, as they lock into a crystal, are forced into a rigid open lattice that holds them further apart than the jostling of the liquid does.

The consequence is not a curiosity, it is the reason there is anything alive in a cold lake. Ice forms on top and then insulates the water beneath it, so lakes and seas freeze downward slowly and usually not all the way. If ice sank, each sheet would drop to the bottom and the next would form on top of it, and bodies of water would freeze solid from below and stay that way through the summer.

The same expansion is a destructive force. Water seeping into a crack in rock and then freezing exerts enormous pressure, which is the main mechanism by which mountains are broken down into scree, and why potholes appear in roads after a hard winter. It is also why a bottle left in the freezer splits, and why frozen pipes burst - the ice does not merely fill the pipe, it pushes the walls apart.