http://www.youtube.com/watch?v=JWToUATLGzs #RIDDIM and #BASE
If you place 32 metronomes on a static object and set them rocking out of phase with one another, they will remain that way indefinitely. Place them on a moveable surface, however, and something very interesting (and very mesmerizing) happens.
The metronomes in this video fall into the latter camp. Energy from the motion of one ticking metronome can affect the motion of every metronome around it, while the motion of every other metronome affects the motion of our original metronome right back. All this inter-metranome "communication" is facilitated by the board, which serves as an energetic intermediary between all the metronomes that rest upon its surface. The metronomes in this video (which are really just pendulums, or, if you want to get really technical, oscillators) are said to be "coupled."
The math and physics surrounding coupled oscillators are actually relevant to a variety of scientific phenomena, including the transfer of sound and thermal conductivity. For a much more detailed explanation of how this works, and how to try it for yourself, check out this excellent video by condensed matter physicist Adam Milcovich.