When we were kids, my sister and I would play tag a lot. Once, we had been chasing each other for quite some time, and while I was "it", she decided she didn't want to play anymore, and went to go sit in our room to be by herself. I never realized that she had stopped playing, because I was still chasing her. Meaning, there was something in our house that had looked just like her, and it was letting me chase it all around the house. It would run so fast and hide behind walls, and when I caught up, the wall would be empty and I would see it running on the other side of the room. Because I was so young, I never actually realized that a person can't move that fast, or that it wasn't actually her. I think I was playing with it for around fifteen minutes before I got mad that my "sister" wouldn't let me tag her. When I had seen it go behind a wall a few rooms away from my bedroom, I decided to quit, and wen't into the room I shared with my sister. When I saw her sitting there I was in shock. She didn't look like she was just running around, and the wall that "she" had hid was too far away from the bedroom for her to have gotten there before me. When I asked how she had gotten there so quick, she looked confused, and told me she had been there for the past fifteen minutes. I was so scared by the incident that I never told her the truth, that I had been chasing something that looked like her around the whole house. To this day, I wonder what would have happened if instead of me chasing the thing that looked like my sister, it had decided to come chase me instead...