Through this curricular unit, students are introduced to the important concept of density. The focus is on the more easily understood densities of solids, but students may also explore the densities of liquids and gases. Students devise methods to determine the densities of solid objects, including the water displacement to determine volumes of irregularly-shaped objects. By comparing densities of various solids to the density of water, and by considering the behavior of different solids when placed in water, students conclude that ordinarily, objects with densities greater than water sink, while those with densities less than water float. Then they explore the principle of buoyancy, and through further experimentation arrive at Archimedes' principle-that a floating object displaces a mass of water equal to its own mass. Students may be surprised to discover that a floating object displaces more water than a sinking object of the same volume.