This demonstration starts with two equal volume blocks of clay and a mallet. The shape of one of the blocks is then hammered with a series of repeated strikes from the mallet to obtain a pancake shape. Explain to the students that the volumes are still the same. The effect is more drastic and entertaining with more strikes.
Note: the mallet strikes need to be hard, requiring a certain level of physical strength. The strikes are also quite loud, so it's a good idea to warn students (unless you don't want to!)