A pendulum hanging off another pendulum turns perfectly predictable physics into chaos
A single pendulum swings in a simple, repeating arc you can predict with one short equation. Attach a second pendulum to the end of the first, and the motion becomes chaotic: two double pendulums started a hair's breadth apart will swing in wildly different, unrepeatable paths within seconds, a textbook demonstration of how deterministic rules can still produce unpredictable behaviour.