Water is one of the few substances on Earth that is less dense as a solid than a liquid. As ice, water molecules form four hydrogen bonds that lock them into a rigid crystalline structure. In this state, the water molecules are actually further apart than when they are in a liquid state. This is why water expands as it freezes and is less dense than the surrounding liquid water. Because solid water is less dense, ice floats on the surface of a lake in winter and insulates the water below from freezing, providing a vital benefit to aquatic organisms. If water in its solid form was denser than water in its liquid form, lakes and ponds would freeze solid to the bottom during winter, and no longer provide viable habitats.
As it is, fish and other species in polar and temperate regions have evolved to live in lakes when outdoor temperatures fall below the freezing point of water (32ºF, 0ºC). These aquatic organisms possess a relatively wide range of tolerance for temperature changes throughout the year. However, the range of tolerance that species have adapted to is being threatened by temperature shifts induced by global climate change. More will be said below and in Chapter 6 about the effects of global climate change on water.