Why does water expand when it's frozen?

Answer 1

The answer lies in the crystal lattice structure theory, in this case Ice: the atoms form a three dimensional structure called a tetrahedron. In this structure the molecules of water make a repeating arrangement of eight water molecules, which is pretty spacious thus water expands on freezing and contracts on melting.

Answer 2

It expands because the forces holding the molecules together are altered, thus setting up a lattice structure where the water molecules are farther apart than when in the liquid state. The density of ice is thus less than that of liquid water (that's why ice floats), and thus the same mass now occupies a greater volume.