Why does ice float on water?

Because freezing locks water molecules into an open, hexagonal crystal. In that arrangement the molecules sit further apart than they do in the liquid, so the same mass of ice occupies more space — so ice is less dense than the water it sits in.

Hydrogen bonding is the reason. Each water molecule can form weak bonds with its neighbours, and on freezing those bonds hold the molecules in a fixed, roomy arrangement. In the liquid the bonds are constantly breaking and reforming, so the molecules can pack more tightly.

The property matters for life. Lakes and rivers freeze from the top down, and the floating ice layer insulates the water beneath. If ice sank, water bodies would freeze from the bottom up and the life in them would not survive.

It is also why a full bottle of water bursts in the freezer — freezing water takes up more room and the expansion cannot be stopped. The same thing happens in nature: water in a crack freezes and splits the rock, which is one of the main ways soil gets made.