Water Shinks slightly when its temperature is raised from 0 degree celcius to 4 degree celcius, but expands for temperatures higher than 4 degrees celcius. The shrinkage occurs when the temperature vavries from 0 to 4 degree celcius actually prevents the lake/pond from freezing completely in winter. That is because the shrinkage means that cold water is slightly more dense than warm water, and so when the pond/lake begins to freeze, the cold water at the surface sinks before turning to ice, allowing slightly warmer water to take its place. After all the water in the lake/pond has reached 4 degree celcius, the water at the surface freezes and becomes ice first because they are in direct contact with the cold air and they floats on the denser water below it.

The layer of ice blocks the cold air over the lake/pond from freezing the water below the ice layer. In other words it acts as an insulator. And that is why lakes never freeze completely, otherwise all the fish and other aquatic organisms are dead in winter!! This is actually important for those aquatic organisms because in winter no birds can hunt them because of the help of the ice layer! (However we human can't still hunt them because we know to drill a hole to fish...)