Judy Krakalovich, age 12, of St. Catharines, Ontario, Canada, for her question:

WHAT IS DENSITY?

The density of a substance is the amount of mass, or matter, it contains for each unit of its volume. The density of a substance is calculated by dividing its mass by its volume, or the amount of space it occupies.

Most substances expand in volume when they are heated. Therefore, the density of most substances decreases when they are heated. Water is an exception. It contracts, or takes up less volume, when it is heated, and its density increases.

Most substances contract when they freeze so that the density of the solid is higher than the density of the liquid. Again, water behaves differently. It expands when it freezes and its density decreases.

Ice floats on water because lower density substances float in liquids of higher density.