Step 1: What You Will Need...
1. A cheapie click pen
2. Some Strike-Anywhere matches (the ones with the white tip only)
3. A bobby pin
Step 2: Disassemble the Pen...
Unscrew the pen and remove the ink insert. Save the spring, that's the firing mechanism. And, save the screw top you first removed.
Step 3: Build the Firing Mechanism...
To build the firing mechanism:
1. Take the spring and slide it over the end of the bobby pin. (Be sure to remove the little plastic protectors on the end of the bobby pin if yours has them)
2. Insert the screw top portion of the pen over the bobby pin.
3.Take a pair of needle nose pliers and bend back a portion of the bobby pin to hold the spring and top in place.
Step 4: Insert the 'ammo'...
Drop a Strike-Anywhere match into the pen body and screw the pen top back in place, exposing the trigger end of the bobby pin.
Step 5: Fire Away!
Pull the bobby pin and it will hit the tip of the Strike-Anywhere match causing it to ignite and spew out a stream of stank sulphur smell...!
(Some testing may require you to 'shim' the match with a small length of wood from another match so the firing mechanism hits the tip of the 'ammo'.)