Water can expand while it is still being cooled.

Should a cooled liquid become ever more compact? Fresh water has an exception: under ordinary pressure conditions, its density reaches a maximum of nearly four degrees Celsius. By continuing to cool it to zero, it becomes less dense. The same volume then contains slightly less mass.

This behavior occurs in lakes. Water close to four degrees can be placed under colder, less dense water. The formation of ice adds another peculiarity: the molecules are organized in a more open structure than that of the liquid. At equal mass, ordinary ice occupies more volume, which explains its buoyancy.

The ice cube of a glass illustrates an important property beyond the kitchen. We must specify which water we are talking about: dissolved salts and other parameters change the density. The rule does not translate to all waters and pressures. Behind a familiar number is unusual behavior, capable of influencing an entire aquatic environment.