So many of our planet's forms of life are dependent on an environment in which liquid water is stable. This means that the earth must not be too close or too far from the sun. Astronomers estimate that if the distance from the earth to the sun changed by as little as 2 percent, all life would be extinguished as water either froze or evaporated.
Another factor making life on earth possible is an unusual characteristic of water when it freezes into ice. Water is such a common substance that most of us do not stop to consider that the balance of life depends on its simple physical properties.
Water is one of the few substances that expands when frozen. Most substances when frozen become denser and sink when placed in a container of the same substance in liquid form. But this isn't the case with ice in water. Since water expands by one tenth its volume when frozen, frozen water has the unusual characteristic of floating on top of liquid water.
When rivers and lakes freeze in the winter, they freeze at the surface, with the ice forming an insulating barrier that prevents the denser water underneath from freezing and thereby preserving aquatic life during very cold weather. If ice acted like almost all other compounds, it would sink, and rivers and lakes would freeze from the bottom up. All bodies of water would eventually become solid bodies of ice, eliminating most life as we know it.