The steps by which water freezes are what makes water very important biologically. A property of water that is especially important for life in cold climate is that it attains maximum density at 4°C,just above the 0°C freezing point.
Therefore as water begins to drop in temperature when its surface is cooled, the water at the surface will reach this highest density. The surface water will then begin to sink, displacing the warmer water below it. This results in freezing of water from the surface downward.
The effect takes place in ponds and lakes that are located in cold climates. Their surface freezes, and the water underneath remains liquid protecting the organisms that live there.
