A coin penetrates a fully transparent, closed container!
Borrow a coin from a spectator, and have it signed if needed. Introduce a transparent, egg-shaped container and have it examined. The magician shows that there is nothing in the container, and closes it shut.
The spectator can see right through the container to be assured. The magician holds the coin on his/her finger and slowly brings it towards the bottom of the container. The moment the coin touches the container, the coin penetrates right through! The magician shakes the container to prove that the coin is really inside. The magician opens the lid and takes the coin out, which can be handed out for examination once again.
The coin is truly the spectator’s signed coin!