Tie the inertia ball with a light string to a heavy ring stand. Tie a heavier string to the bottom of the ball. Have 2 strings attached to the ring stand so the demo can be done twice. First, pull slowly from the bottom with the heavier string. The light string above the ball will break before the bottom string will. Second, attach the inertia ball from the second light string above and pull on the lower string with a quick, sharp jerk. The lower, heavier string should break before the lighter string above breaks because of the inertia of the heavy ball.