Density is often measured by whether an object floats or sinks in water. Real Japanese 1 Yen coins are made from aluminum and can be made to float on the surface of a cup of water. Giant Pumice is the largest volcanic pumice sample we have ever seen... and it still floats! We have super dense wood that sinks in water. Powered by baking powder, the Diving Submarine dives and surfaces just like a real submarine.