What it shows: Fifteen uncoupled simple pendulums of monotonically increasing lengths dance together to produce a series of different waves and motion.  Aliasing and quantum revival can also be shown. 

How it works: The period of one complete cycle of the dance is 60 seconds.  The length of the shortest pendulum has been adjusted so that it executes 51 oscillations in this 60 second period.  The length of each successive shorter pendulum is carefully adjusted so that it executes one additional oscillation in this period.  Thus, the 15th pendulum undergoes 65 oscillations. When all 15 pendulums are started together, they quickly fall out of sync - their relative phases continuously change because of their different periods of oscillation.  However, at 60 seconds they will have executed an integral number of oscillations and be back in sync again at that instant, ready to repeat the dance. 

Video and text from Harvard National Sciences Lecture Demonstrations.