Harvard’s Natural Sciences Lecture Demonstrations team produced a remarkable video showing fifteen uncoupled pendulums. The pendulums were designed so as to have frequencies of 51 – 65 swings per minute. Over the course of a minute they go in and out of phase with each other. You’ll see them at various points beating as three groups of five or five groups of three. At the halfway point, half swing out of phase with the other half. See for yourself:
The folks at Harvard credit Richard Berg at the University of Maryland with the design. He apparently saw it at Moscow State University, but the ultimate origins of this clever demonstration are a bit obscure. Feel free to add a comment if you have more information.