From the "I Wish This Were My Desk Toy" department, here's a video of a wave pendulum built using billiard balls. It's oddly hypnotic, as you watch the series of individual pendulums acting on their own (swinging to and fro), but as a group they appear to interact, forming intricate patterns, syncing up and breaking apart.
Joe Hanson of It's Okay To Be Smart explains:
A wave pendulum like this is built of equally weighted objects suspended by different (and carefully calculated) lengths of string. Released simultaneously, their differing periods (frequencies of oscillation) cause them to form a “wave” image together that cycles through all possible patterns. It’s mind-boggling. Looking for a science fair project? This would be a great one.
And here's a similar wave pendulum video, minus the colored billiard balls:
And one with a black light:
For the actual physics behind this, read a detailed analysis from SuperFly Physics.
(Via The Kid Should See This.)