Why does water expand when it is frozen?

Water behaves differently than almost all other liquids because it expands when it is frozen. As a liquid is cooled down the molecules move more slowly allowing the attractive forces between them to become stronger. Eventually, these forces become strong enough to hold the molecules in a fixed position (they can still vibrate but not change their relative positions) and the liquid becomes a solid. The molecules in the solid have reached an optimum 'closeness' …

…bond is at least 10 times stronger than the other types of attractive forces that hold molecules together. The maximum number of hydrogen bonds a water molecule can form is 4 and in ice each water molecule is doing just that. Water expands to around 9-10% its original volume. So the attractive forces are maximized even though the molecules are forming a very open lattice structure, causing the water to expand and form a block of ice.