How do u do this trick?

My younger sisters told me about a thing that u have to do in a room with all lights closed looking at a mirror. U say 'bloody Mary' x3, and then u stare in your face and then it starts changing. It works for them and their friends but not me. They are saying my face is changing but I'm just not creeped out by it. How the fuck are u supposed to do it lol?