Thermodynamics

Water has the usual property of contracting when the temperature decreases, but only down to 39.2°F (4°C); below that temperature it expands until it reaches 32°F (0°C). It then forms ice at 0°C, expanding considerably in the process; the ice then behaves "normally," contracting as the temperature decreases. Since the density of a substance varies inversely to the volume (as a given mass of a substance expands, its density decreases), this means that the density of water increases as the temperature decreases until 4°C, when it reaches its maximum density. The density of the water then decreases from 4°C to 0°C; the formation of the ice also involves a decrease in density. The ice then increases its density as its temperature falls below 0°C. Thus, as a lake gets colder, the water at the top cools off and, since its density is increasing, this colder water sinks to the bottom. However, when the temperature of the water at the top becomes lower than 4°C, it remains at the top since its density is lower than that of the water below it. The pond then ices over, with the ice remaining at the top, while the water below remains at 4°C (until, if ever, the entire lake freezes). Fish are thus able to live in lakes even when ice forms at the top, since they have the 4°C water below it to live in.