Harvard students built a device with a series of 15 uncoupled simple pendulums of monotonically increasing lengths. They then set them in motion and the result is the pendulums dancing together to produce visual traveling waves, standing waves, beating, and (seemingly) random motion.
The period of one complete cycle of the dance is 60 seconds. The length of the longest 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 (shortest) undergoes 65 oscillations.
This apparatus was built from a design published by Richard Berg [Am J Phys 59(2), 186-187 (1991)] at the University of Maryland. The particular apparatus shown here was built by Nils Sorensen.