Five small silver balls hang in a perfectly straight line by thin threads that attach them to two parallel horizontal bars, which are in turn attached to a base.

If you pull a ball up and out and then release it, it falls back and collides with the others with a loud click. Then, instead of all four remaining balls swinging out, only the ball on the opposite end jumps forward, leaving its comrades behind, hanging still. That ball slows to a stop and then falls back, and all five are briefly reunited before the first ball is pushed away from the group again.