The common form of ice shown on the last page has open, cage like channels running through the structure. Ice indeed is an open framework structure, and the water molecules can get closer together if they break the framework and pack together in a more random manner in a liquid. This is the reason why ice is less dense than liquid water, and floats at the surface of a lake. With the exception of a few bismuth-cadmium alloys used in making printer's type, no other liquid expands upon freezing, and no other solid floats on its own liquid. This is an important property of water for life on Earth, for if ice sank to the bottom as it froze, then the bottom of the world's oceans would be perpetually frozen, with the melting boundary rising in winter and falling again in summer. The ocean floor would be covered with a permanent layer of ice. With the coldest water at the bottom next to the ice surface, there would be no convection currents and no mixing of materials in the ocean. One can pursue this line of thought and predict that in such a world, life probably never would have evolved. It is tempting to speculate that, on this basis alone, life wherever it is to be found in the universe probably will be associated with water-bearing planets.