Wrap very thin filament wire (high resistance, low flash point) around the fuse of an M80 firecracker. Imbed this assembly in a box of cornflakes, leading the wires out of the box.
Next, wire several old-fashioned camera flashbulbs in parallel, and put them in a circuit with the firecracker wire and a 12 V lantern battery. Rig a solenoid to close the circuit (we used an old clothespin).
Now you're ready. When the victim is out of his/her room, hang the cornflake box in the middle of the room. Arrange the flashbulbs near the door. Place the solenoid so that opening the door will close the circuit. Lounge nearby and watch the fun.
When the victim opens the door, s/he thinks that the room just exploded. Bright light, loud noise, and the burnt cornflakes feel like a bomb-blast concussion as they hit your body.
