The displacement answers of mew and kal are spot on.
This is about the chemistry of the displacement.
Water is the only substance with solid density less than the liquid. (For the record there is one other, namely the element Gallium).
As you cool a liquid and it settles in and get more dense. As it settles to solid it typically just settles in more.
Water is very a interesting molecule in that it is very stable and still polar. Two hydrogen and one oxygen in a triangle. The oxygen is slightly negative and the hydrogen positive. When they settle into a solid they form a lattice that is less dense than the liquid state. Water has a maximum density as 4 Celsius.
If ice was more dense than water we would be on a much much different earth. We would probably not be on this earth.
