Step 3 includes a video summary of the instructions, plus a firing.
Mind the lights.
Step 1: What you need:
A 35mm film cannister - the tighter the lid fits, the better, so try for the translucent white ones. I've gone digital, but I can still get loads of the cannisters by asking nicely at my local Boots photo counter (it saves the staff walking out to the recycle bin).
The sparking-unit from a cigarette lighter. I'm not a smoker, but they're convenient for lighting Bunsen burners. This one, though, was actually confiscated from a pupil caught shocking people in the corridor.
A sharp knife - penknife, Xacto, anything that can make short, clean cuts in the cannister. I would not recommend using a kitchen knife - the relatively long blade makes them harder to control in short, fine cuts, and more dangerous if you slip.
Fuel. Some people use hair-spray, SciToys uses mouth-freshener spray, but my fuel of choice is methanol (denatured alcohol, available in the US as camping stove fuel or Heet).
As an option, you may want to fix the sparker in place. Several different glues will do the job - Serious Glue, hot glue, contact adhesive.
Step 2: Making the Cannon.
Other versions fire the cannister, this one fires the lid.
Near the bottom of the cannister, cut a small hole that is just large enough to fit the sparker's button through.
Bend the wire of the sparker over towards the larger contact and make sure it works - press the button and watch for the spark.
Slot the sparker through the hole from the inside of the cannister. If your fingers are even slightly on the chubby side, you may find it easier to use a pair of long-nosed pliers or a pair of forceps to do this. Try not to move the wire too much. Check that the wire is still in the right place to strike a spark.
There you go, you've finished - don't add any glue until you have checked the cannon works.
Step 3: Firing.
Really, just a few drops. Five or six is plenty.
Put the lid on, stand the cannister on a firm surface, and press the plunger once or twice.
With the original fuel-load, you may be able to get two, or even three separate shots, each as loud as the last.
Health and Safety
When it fires, he cannon is LOUD. If you are indoors, your ears will ring - you may choose to wear ear-defenders.
The lid comes off fast - keep your face clear, wear goggles and never, ever point it at anybody.
Be aware of the fire-hazard. Whatever fuel you choose to use, it will (by its very nature) be highly flammable. If you use methanol, remember that it burns with an almost-invisible flame. Check carefully that excess fuel is not burning before you put the cannon down.
Step 4: At the Thanksgiving Table.
Why don't you make enough porta-cannon for the whole family? If you use methanol, they can be fuelled up in advance (store them in an air-tight container to stop the smell ruining the meal). Hand them round at just the right time, describe the firing process and then count down ... 3 ... 2 ... 1 ...
Just make sure the family pets are out of the