"How well can a hamster vacuum a room? The Create offered a great platform for implementing this silly idea. The hamster controls the robot by running in the ball in whatever direction he/she likes. The ball is supported on three rollers and an optical sensor measures which way and how fast the ball is spinning. Those values are fed into a microprocessor that controls the drive wheels to make the robot mimic the motion of the ball."

From http://www.neatorama.com/2009/02/26/hamster-controlled-roomba/ ...