The mentalist displays seven keys and a padlock, explaining that only one of the keys will open the padlock. The keys are dropped in a bag and seven spectators are each given the opportunity to choose a key (well, technically, the seventh one has no choice, but we’re going to avoid getting too picky here).
The mentalist waves a hand over each spectator in turn, and then accurately divines which spectator has the one working key. The lock is undone, the cage door opened, and Baldpate the Gorilla escapes into the audience for a thrilling climax.
The lock, keys, cage, and gorilla are all real. The mentalist knows ahead of time which spectator has the correct key because the key that will open the lock on the gorilla’s cage is brass while the rest are silver (the body of the lock is also brass, to help the mentalist remember what color to look for). The mentalist just watches for the person who picks the odd-colored key, and the rest is acting.
Some mentalists use seven keys that are all the same color and memorize the pattern of teeth on the keys to make the method even more inscrutable. This seems like a lot of extra work for very little gain, as the risk of discovery is slight. Spectators never notice the colors of the keys because they are too distracted by the wild thrashing of the mad gorilla within its cage (not feeding the gorilla for a few days and putting a banana under a spectator’s chair enhances this effect).