The Embodiment of Chaos
A single pendulum is boring. It swings back and forth, forever predictable. You can set your watch by it. Attach a second pendulum to the bottom of the first, and you break reality.
Why Is It Unpredictable?
The system is governed by regular Newtonian physics ($F=ma$), but it is highly sensitive. If you release the pendulum from an angle of $90.00°$, it traces one path. If you release it from $90.01°$, it traces a path that looks identical for about 5 seconds... and then wildly diverges.
This is the physical demonstration of the "Butterfly Effect." You cannot measure the starting position accurately enough to predict the position 2 minutes from now. The error margin explodes exponentially.
Energy Transfer
Watch how the "elbow" joint locks in place while the "arm" spins wildly, then suddenly the arm stops and the elbow flails. The energy transfers kinetically between the two limbs in a hypnotic dance.