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