you should paint creepy shapes in glow in the dark paint on the walls, then paint over it in a watered down coat of the normal color(works well with really light phos paint and a thin coat of white ), so once the light are turned out, there will be shapes on the walls that fade after a minute, then if they turn the light on for only a few minutes they won't be there, but if they keep it on for a while they'll reappear