Cool physics experiment video with metronomes

Man takes five metronomes ticking randomly. Puts them on a slightly mobile plank held on two tin cans. Metronomes run in sync. Magic.
http://www.youtube.com/watch?v=W1TMZASCR-I

The earliest known scientific discussion of synchronization dates back to 1657 when Christian Huygens built the first working pendulum clock. Huygens studied systems of two pendulum clocks mounted on a common base. He observed that the clocks would swing at the same frequency and 180 degrees out of phase. This motion was robust, after a disturbance the synchronized motion came back in about half an hour. ...

As one metronome's pendulum bob moves to the right, this pushes the base to the left (because of momentum conservation). The base moving to the left then pushes the other metronome's pendulum bob to the right---i.e. in the same direction as the first pendulum. Thus the slightly faster pendulum gives a kick (through the base) to the slower metronome causing the slower pendulum to speed up.
More: http://salt.uaa.alaska.edu/dept/metro.html

A phase-locked loop or phase lock loop (PLL) is a control system that generates a signal that has a fixed relation to the phase of a "reference" signal. A phase-locked loop circuit responds to both the frequency and the phase of the input signals, automatically raising or lowering the frequency of a controlled oscillator until it is matched to the reference in both frequency and phase. A phase-locked loop is an example of a control system using negative feedback.
http://en.wikipedia.org/wiki/PLL