A mentalist plants his phone lock screen into a spectators mind. This is a mentalism effect that lets you telepathically impart your phones lock code to a random stranger.
The mentalist walks to a group of people and asking if they can help him with an experiment of the mind. All spectators try to unlock the phone but no one can do it, except the chosen one that the mentalist chooses to perform the telepathy technique. And this person is the only one who can unlock the screen.

Highly recommend Android KitKat version at minimum.