Water behaves in an unusual way when it is heated from 0°C. As the temperature rises from 0°C to 4°C it actually contracts. However, from 4°C upwards it expands like any other liquid.
This means that water takes up the least space at 4°C. It has the highest density at this temperature and will sink through warmer or colder water around it. The density of water at 4°C is 1 g/m3.
Why it takes months for a lake to freeze while a small bucket of water can freeze overnight on a bitterly cold day?
The surface water cools down to 4°C and sinks to the bottom of the lake due to its high density and hotter water comes up to the surface. Gradually the whole water cools down to 4°C. Further cooling decreases the temperature of surface water which finally freezes. Ice being lighter than water keeps floating on the surface. It acts as an insulator and slows down the cooling and freezing of the lower layers of water.
This explains why aquatic animals living in water bodies of very cold regions do not die in severe winter.