On the 13th day of any month, find a hand-held mirror, just large enough to cover your face. Cover your face with the reflective side out, walk into the bathroom, turn the light on, and stand in front of the larger mirror. At exactly 12:00am, slowly raise the hand-held mirror above your head.
What is in the larger mirror will not be staring back at you, but nor will it be your reflection.
Very carefully walk out of the bathroom, backwards, not moving the hand-held mirror until the one in the bathroom is completely out of view.
If you do not, what you saw in the mirror will notice, and realize what you have done...