A tennis ball (60g) and a pool ball (200g) are connected with a low-mass but very rigid carbon fiber tube, 20 cm center-to-center.  Balance the assembly on your finger or on a triangular pivot stand.

A second version of this demo is available that has a tennis ball (60g) on one end and a lead shot filled tennis ball (550g) on the other.  Since this makes the balance point very close to the heavier ball, a special triangular pivot stand is needed.