Sharon Sassaman, age 11, of Allentown, Penna. for her question:

Most substances shrink as they reach freezing point and become solid. They become more dense, heavier and more solidly packed. Water also becomes heavier and more dense as it cools. That is, until it reaches the temperature of four degrees Centigrade. This is four degrees above freezing At this point the process towards weight and density changes. The solid ice forms it is lighter and less dense than liquid water. It also takes up more room.

This happens because the crystals of ice form in an open framework. If we could see the tiny particles of which it is made we would notice spaces among them. These spaces give volume and lightness to the ice.

Ice floats, of courses because it is lighter, less dense, than water. What a world this would be if this were not so. If ice were heavier than water it would form on the bottom of rivers and oceans. The sun could never melt it. Soon the rivers and seas would fill with ice from below. There would be liquid water only on the surface of the waters.