well i made it with my step father when i was younger. but here it gos. DONT TRY THIS UNLESS YOU KNOW WHAT YOUR DOING.
First you get yourself a long pipe. Ours was this realy complacated pvc pipe with built in lighter. But The easest way to make one is just get yourself some pipe. Get something to plug up the end.
Drill a hole in the end you plugged up. not on the plug itself but above it. on the top of the cannon. We had this little spark maker that we used to set off the explosive but i think you can use any thing that is on fire. A match will work.
now you have your self a pipe, pluged up on one end, with a hole drilled on top. A small hole just big enough for the match or what ever. Get yourself something that is flammibul and has vapors. I like too use hair spray it works the best.
Spray the inside of the cannon. Too much and youll blow yourself up and too little the potato wont fly out. Put some wading in if you like, we did not, then insert the potato. It needs too be small enough that it fits snug in the cannon. If its too small use some wading. Tissue paper works well.
Touch the match to the hole, or drop the match in, you hear a loud rore and ether theres a flying potato or your on fire.
like i say not safe but fun