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Most things shrink when they freeze, but water does the opposite. As it turns to ice, its molecules lock into a tidy pattern with gaps in it, a bit like people holding hands in a ring instead of crowding together. The same water takes up more room as ice, so ice is lighter for its size than the water around it. That is why ice cubes bob at the top of your drink, and why ponds freeze from the top down.

Ice floats because it is less dense than liquid water: about 0.92 grams per cubic centimetre, against roughly 1.00. Each water molecule can hydrogen-bond to four neighbours, and in ice those bonds hold the molecules in an open hexagonal lattice. In the liquid, the bonds keep breaking and re-forming, so molecules can pack more closely. Liquid water is densest near 4 °C, so in winter the colder water stays at the top of a lake and freezes there, while the deep water stays near 4 °C.

Ordinary ice (ice Ih) has a density of about 0.917 g/cm³ at 0 °C, some 8% below liquid water at the same temperature, so it occupies about 9% more volume. Its oxygen atoms form a hexagonal lattice in which each is tetrahedrally hydrogen-bonded to four neighbours, with the protons disordered according to the Bernal–Fowler ice rules. On melting, the network partly breaks down: molecules move into the cavities of the open lattice and each gains more near neighbours, so density rises despite greater thermal motion. The competition between this local tetrahedral order and ordinary thermal expansion gives liquid water its density maximum near 3.98 °C at atmospheric pressure. The rule is not universal: high-pressure phases such as ice III, V and VI are denser than water and would sink in it.