The spectator is handed an open brass combination padlock. They are instructed to remove one of their finger rings, drop it onto the clasp and then close the lock by turning the combination randomly.
After mixing up the combination the spectator is asked to try and re-open the lock by guessing its unique four digit combination. After several failed guesses, the mentalist instructs the spectator to enter their exact year of birth into the lock.
Despite the seemingly heavy odds of one in ten-thousand, everyone is amazed to find that the lock now opens up on this exact combination!
As an alternative approach, you may wish for the lock to open up on the first four digits of the spectators mobile number. This custom gimmicked lock does all of the work for you and opens up countless routining possibilities.
Whilst this lock is small enough to carry on your keyring, it can also play to larger audiences by perhaps using it to lock a larger box (maybe containing your final prediction thus adding an extra element to the closer of your show!
Note: The mentalist need not touch the lock at any point during the effect!