At one of the websites I frequent the first paragraph of this article was posted.
I guessed the teacher had set up the plate and turned it around.
As a student of the theatre I am somewhat versed in the arts of "Illusion". There was one show where we set off a smoke bomb and lowered the lights at the end of the first act. We then had intermission At the beginning of the second act we set of a smoke bomb and raised the lights. It's amazing how many people wrote about how we made the prop appear out of nowhere on the stage. They edited the intermission out of their remembrance of the story.
This story quickly sprang to my mind and I realized that if you can fool someone into forgetting an intermission you can fool someone that had no idea what was done before they came into the room in the first place.