When a tennis ball loses its bounce, upcycle it into a useful hand weight and piggybank -- at the same time! Use the point of a sharp knife to make a small slit on the side of the tennis ball. Fill the ball with spare change; the more coins you add, the heavier it will be. When full, you have a useful weight perfect for exercise. To prevent coins from spilling out, you can add a piece of duct tape over the slit to create a secure closure.