Water shows an unusual or anomalous behaviour when it is heated or cooled between 0°C and 4°C. All substances generally contract on cooling while water expands.
When water is cooled, it first contracts just like other liquids but upto 4°C. On further cooling, it expands instead of contracting. This expansion takes place upto 0°C. Thus, at 0°C, water has maximum volume and minimum density.