Just had an idea while I was on the john... maybe a good device to patent and sell... a cell phone jammer. The use case that comes to mind is a university that wants to prevent cheating. They could either have materials in the classrooms that prevents cell phone waves from getting in or the teachers could each have a jammer that has a specified radius that it jams and they could place it in the center of the room, that way they could turn it on and off so that emergency situations could still get through during non-testing times.