Why does ice float over water?

We know that in ice, each oxygen atom is surrounded by four hydrogen atoms in such a way that the two hydrogen atoms are linked to an oxygen atom by covalent bonds whereas the other two hydrogen atoms are linked by hydrogen bonds. In ice (solid state), a water molecule is associated with four other water molecules through hydrogen bonding in a tetrahedral manner.

This gives rise to open cage-like structure which prevents the close packing of molecules (lower density). When ice absorbs heat and melts to form water, the hydrogen bonds break and close packing of water molecules takes place. Due to this close packing, the density of water is higher than that of ice and hence ice floats over water.