In the 1600's Huygens noticed that clocks (pendulums) that were loosely coupled would fall into synchronization. It should be pretty easy to demonstrate this:

• Clamp a meter stick on edge between two supports.
• Clip a few equal length chord and weight pendulums along it
• Set them swinging at random
• Watch as they synchronize

But:

• What if the lengths aren't exactly the same? ('cause they won't be)
• Does it matter where they are clamped with regards to each other or the supports?
• Most demos use pendulums swinging in the same (or nearly the same) plane, but this idea would have them swinging in parallel planes. Does this matter?