Three lightweight balls hang in-line on a single string attached to a motor.  A switch mounted on a dashboard of a floor consul controls the speed of the motor.
The balls arrange into several distinct formations as the motor changes speed.  The transition periods between the stable formations seems almost magical.

Versions of Three Balls on a String have been purchased by:

Phaeno Museum, Wolfsburg, Germany.
Alaska Science Center, Anchorage.
The Don Harrington Discovery Museum,  Amarillo, TX.
The Exploratorium,  San Francisco, CA.