Pendulums are nice. They are simple enough that we can describe their motion easily while also being complex enough to surprise us. At small angles we can find the analytical solution for a single pendulum easily enough, but as we increase the angle determining its trajectory is far from trivial. Adding in a second pendulum to the end of the moves us into the realm of chaotic dynamics, where a small change in initial conditions can impart a major change in the trajectory.
By clicking on either of the links below you can explore the dynamics of the single or double pendulum.

Single Pendulum

Double Pendulum