My older sister loved to torment me when I was little. One day she told me she had a secret to tell me and I was extatic. So I went with her into the bathroom and she turned the lights off and said if you look into the mirror with the lights off and say 'Bloody mary.' Three times in a row she will come. I thought that bloody mary would just start killing everyone in the world and it scared me. So of course my sister used this to her advantage and tried to get me to do things for her and everytime I said no she would run into the bathroom turn off the lights stare at the mirror and start saying 'Bloody mary, bloody mary..' Before she could finish saying it the second time I would either be crying and agreeing to do it or practically jumping on her and agreeing to do it.
A childhood belief about ghosts
