Here a swing is arranged with an independent pendulum. First oscillate the pendulum from left to right direction. Then oscillate the swing from front to back direction. Observe the pointer’s movement at the bottom of the pendulum.

What did you observe? 
The pointer moves in a regular pattern. Although the pendulum here can move only in a single plane, it appears to be moving in three dimensional plane.

The science behind it :
Here, the swing and the pendulum are acting as two simple pendulums at fixed phase difference having two different simple harmonic motions. When we oscillate them one by one, the resultant of two simple harmonic motions create a uniform pattern traced by the pendulum end.