We've improved the density experiment in a dozen ways: We've added two cubes to our 10-cube set. Lignum Vitae (ironwood), one of the heaviest and hardest woods in the world, sinks in water, but because it is wood, your students may expect it to float. Polyproylene floats in water, but because it is dense plastic, your students may expect it to sink. 12 one-inch cubes, with instructions and handy storage case.