The spectator is handed a an open brass combination padlock and is asked to link their ring onto the lock and snap it closed. The combination is mixed up and the spectator is asked to try and guess what it is! Unfortunately they don't have much luck...but the performer comes up with an idea! He asks the spectator to put their year of birth into the lock...and amazingly when they do the lock snaps open.
This specially gimmicked padlock does all the work for you...you don't need to know the spectators year of birth, in fact, any date will work! The number doesn't have to be a year of birth, it could be the first four digits of a phone number, an important year to a spectator (more personal!)...anything you want!
The incredibly clever method to this trick leaves it open to a range of possibilities for both magic and mentalism.