Final answer:
Water expands as it cools below +4°C and freezes at temperatures below 0°C. The layer of ice on the surface of water acts as an insulator.
Step-by-step explanation:
In general, objects expand with increasing temperature. However, water is an exception and expands as it cools between +4°C and 0°C (40°F to 32°F). Water is densest at +4°C. When water near the surface of a pond cools to 4°C, it sinks to the bottom, creating a layer of warmer water near the surface. If the temperature drops below 4°C, the water at the surface will stay near the top and freeze, forming a layer of ice that insulates the liquid water below. This property of water allows fish and other aquatic life to survive beneath the ice.