You have a borrowed banknote. Ask a spectator to sign it. You put it in an enveloppe and you burn it. Take a brass tube from your pocket ang give it to the worried spectator. The is locked with a padlock so show him the keys on the table. He open the tube and...he find the banknote inside ! Each parts of the equipment can be examined.
Faites signer le billet par son propriétaire : il sera d'autant plus content de le retrouver.