Check out this amazing homemade pendulum wave made with 15 billiard balls. More scientifically, what you are seeing is:

Fifteen uncoupled simple pendulums of monotonically increasing lengths dance together to produce visual traveling waves, standing waves, beating, and random motion. One might call this kinetic art and the choreography of the dance of the pendulums is stunning! Aliasing and quantum revival can also be shown. [source]

 

If you want to delve deeper into the science of what you are watching check out this post from Harvard Natural Sciences Lecture Demonstrations and this post from Arbor Scientific which includes a little mathematics.