Five metronomes started at different times eventually synchronize in this nifty video from UCLA. It's been a known phenomenon since the 17th century:
In 1665, Dutch physicist Christiaan Huygens, inventor of the pendulum clock, was lying in bed with a minor illness and watching two of his clocks hanging on a wall, said Henrique Oliveira, a mathematician at the University of Lisbon and co-author of a new study detailing the findings. Huygens noticed something odd: No matter how the pendulums on these clocks began, within about a half-hour, they ended up swinging in exactly the opposite direction from each other.
Image: YouTube / uclaphysicsvideo
