Dancing Pendulum Snake
I’ve already watched this several times, it’s mesmerizing. There’s a beautiful, inaudible music displayed in the patterns if you imagine each length as a different tone or instrument. After watching the unit as a whole, try to focus on one and note how the others operate around it.
As BadAstronomer Phil Plait points out, a pendulum’s period is determined not by the mass at its end, but the total length of the pendulum itself. The pattern that emerges from the variation of lengths across these 15 pendulums is a great example of complexity arising from simplicity.
A helpful commenter there also linked directions to make your own (pdf warning).