A solid brass plinth is displayed along with a gleaming steel ball. The ball is placed atop the plinth and slowly it sinks into the solid brass column. Just as mysteriously it rises again to the surface.This may be repeated, with the ball going even deeper into the solid mass of brass. Finally, the ball is removed and everything can be examined.
Remember, nothing is added or taken away. Everything is done in full view, and all may be examined.