Water expands when it freezes and reaches a maximum density at about 39 degrees Fahrenheit; bodies of water freeze on the top first. The hydrogen and oxygen molecules transfer heat to the air around them, making them bind more tightly but then change to hexagonal form and expand.
Icebergs can float because water freezes at the top first. The expansion of water upon freezing happens because water crystallizes into an open hexagonal lattice form. This form contains more space than water does in a liquid state. At ordinary temperatures, water contracts and increases in density at about one part in 10,000 as it cools.