Water is an amazing substance, it is the only thing known to get bigger when it freezes.  Remember
we learned earlier that when something becomes a
solid the molecules slow down and become
attached to each other.

In a covalent bond in water (hydrogen and oxygen) the oxygen atom has more attraction for the
electrons than the hydrogen does, this makes the electrons orbit around the oxygen more than the
hydrogen atoms.  This makes the molecule
polar.  (it has positive and negative sides)

When water freezes the positive and negative ends join into little hollow balls of atoms (see above)
this is the reason water expands when it freezes and why your ice cube floats in your drink.
Covalent Bonds - More on Water