If nothing collected in the prints, the mud dried and made a mold fossil. If sediments filled the imprint, a sedimentary rock formed with the resulting cast fossil. The powder is heated to dry out all the moisture. This dry powder changes back into a solid when water is added, but it never looks clear and shiny again. It is obvious that the densities of the two are quite different.
Density is a property used by chemists to help identify substances.
It is the amount of mass in a particular volume of a substance. Make sure no air is trapped under the cans. Using the glass container makes a nice visual demonstration for a large group such as your classroom. Use an eyedropper to carefully drip some regular cola through the straw and into the bottom of the water cup. Do not stir the liquids, and notice where the cola stays.