The Basement Window
Inbetween two identical cellar rooms, room a and b, a window was placed. Within the window a lamp was constructed. When the string was pulled in room a room b lit up and vice versa. When lit the participants are unaware of the adjacent room and participants as the window turns from transparent to reflective due to the exposure of the lamp. They instead become aware of their own room and the string becomes visible and the participant pulls it again and the roles are shifted once more.