"What's the density?" may be easy to ask but hard to answer. Here's one way for your entire class to do so quickly and accurately. Twelve specimens look alike but are actually two different plastics. Place in a tub of water. Why do some float and some sink? Have your class graph mass versus volume and analyze the results. Includes 12 samples, 6 each of two plastics; wood holder with linseed oil coating; instructions.