Why does water expand when it freezes?

Ice forms a lattice structure, which occupies more volume than its liquid counter-part.

Lattice structures comprise most of the solids that exist in the world. They are a large complex of like atoms or molecules bonded to each other to form a 3-dimensional figure. Free floating liquid water molecules can get as close as it wants to one another, as the only force stopping them are their own atoms. When it is a solid, however, they are in a fixed position at a specific distance away, a distance which is greater than the spacing of liquid water molecules.