If you place two metronomes (or three, or fife, or one hundred) on a stationary board and start them out of sync, they will remain out of sync (or out of phase) forever. But if the board is placed on two rollers (or empty cans), so that the movement of the metronomes can be transferred through the moveable board, each metronome’s movement will affect the movement of the other metronomes and they will all work their way into sync. The laterally moveable board serves to couple the metronomes to each other allowing the systems (metronomes) to interact with each other. When one metronome’s pendulum bob moves to the right, this pushes the base to the left (the principle of momentum conservation). The base moving left then pushes the other metronome’s pendulum bob to the right. Eventually all metronomes on the board will become synchronized. Biology offers many examples of synchronization: cells in the heart beat together, audiences often applaud together, fireflies flash in synchrony, etc.
In the video below, the man used thirty two colorful metronomes. He put each out of sync and then filmed while they slowly worked their way into synchronization. The visual of dozens of colorful clacking metronomes slowly working their way into synchronization is a beautiful thing to see.