Students make a tin can toy with a secret mechanism. When you roll it away, it comes rolling back!
Adapted from GEM this coffee tin contains a simple elastic powered motor. The "secret" to the Come-Back Can is in the weight that hangs from the rubber band. As long as the weight hangs down as the tin rolls in one direction (and doesn't flip) the kinetic energy of the rolling can will be stored as potential energy in the tightly twisted rubber band (some will be transferred to friction (heat) and sound). Once the band has reached its limit, the stored energy is released and the Come-Back Can moves backwards towards its starting point.