I used them in elementary school. As an observation for those with interestingly placed closets: I used to take the phone (ages ago, when the phone was still on a long cord) and sit in a hall closet where I would be undisturbed when talking to friends. This long narrow closet had a door on either side of it, and the doors had keyholes. The door on the side that went into our library used to have a pinhole effect through the keyhole. I'd sit in the dark closet and look at an upside down image of the library windows on the wall opposite the door.

SRS