5 metronomes on board and cans synch their oscillations

Five metronomes sit on a light foam board that rests on top of two empty soda cans that can roll. The metronomes are set to oscillate at the same frequency, but at the start they are out of sync with each other. After a minute or so, the back and forth movement of the board on the cans causes the metronomes to all sync with each other. Thanks to the Harvard Natural Sciences Lecture Demonstration for the original idea.
UCB Index: 
Demo Diagram: 



The entire setup is kept in cabinet 35-1 in Le Conte. Make sure the soda cans are positioned relatively close to the outside of the setup, close to the outermost metronomes.