This is the only substance in the world, which is compressed after melting firstly, and then begins to extend during the process of increase of temperature. The greatest density water has at +4°C. This anomaly is explained by the fact, that liquid water actually represents a complex solution of water in... the water.
According to one of hypotheses, at fall of temperature lower, than 4°C, something forces single molecules "to move apart" and to become small bricks of ordered structure of ice, increasing volume and reducing density of ice. It is not so clear, what forces them to get out of internuclear holes. It is well known, for example, that pure water
"does not like " to freeze.
