The EKI 2088 Hidden Tormentor Kit can be used as a game, or just to drive someone crazy.
The circuit produces an irritating pitch for about 8 seconds and then shuts down for two minutes. It then repeats...and repeats, etc. until found by the victim.
Components: 6 resistors, 2 capacitors, 4 transistors, speaker, slide switch, battery snap Principles taught: transistors, voltage controlled oscillators