You know how much important matter the water is for life. But you might not know its special property.

Most materials expand when heated or when their temperature is increased but water contracts when heated between \(0{{\kern 1pt} ^ \circ }{\rm{C}}\) and \(4{{\kern 1pt} ^ \circ }{\rm{C}}\). And above \(4{{\kern 1pt} ^ \circ }{\rm{C}}\) it again expands. Water shows its anomalous behaviour only between the temperatures \(0{{\kern 1pt} ^ \circ }{\rm{C}}\) and \(4{{\kern 1pt} ^ \circ }{\rm{C}}\). Since the volume of water decreases continuously from \(0{{\kern 1pt} ^ \circ }{\rm{C}}\) to \(4{{\kern 1pt} ^ \circ }{\rm{C}}\), it has the maximum density at \(4{{\kern 1pt} ^ \circ }{\rm{C}}\).

You can see the ice floating on the surface of ponds or lakes. Because water at \(4{{\kern 1pt} ^ \circ }{\rm{C}}\) has the greatest density, it continuously flows towards the bottom of a lake.

This has a very useful effect on animals who still can survive at the bottom of the lake even if the surface of the lake is frozen. This is the reason why ponds and lakes start to freeze from the surface not from the bottom.