After reading about Eric's potato gun, I sat here and wondered how many people might try to make one and get blown up. There are a few cautions. If you are not a southerner, do not try this at home. Even if you are a southerner, be sure that you don't put too much stuff in there at one time. One spud (I've always called them "spud cannons.") will go twice as far as two. Three or more may cause the gun to explode.
Heavy objects, like a can of soup or a rock, have too much inertia and the pressure builds up too fast in the combustion chamber. It could boom out the wrong end. Don't experiment with fuels. There are some common combustables out there that will end your day quickly.
A safer projectile is a tennis ball. It has a higher mussle velocity, but shorter range. This is due to the low mass and higher air friction. The trajectory is different from a spud, because it tends to fall straight down after it loses its initial speed. Another advantage is that it's a lot less fun being hit by a falling potato. A tennis ball will only hurt at short range, and if you use a high angle, that shouldn't be a problem.
Most cities have laws about discharging a fire arm in the city limits. The definition of a "fire arm" easily fits a spud cannon.
I don't recommend these homemade weapons, but I have to admit that they are pretty exciting at night. Forget the potato and you can enjoy a fireball big enough to bring the cops.
One more thing. If you insist on experimenting with spud cannons, don't forget to yell "Hey, y'all, watch this!" before igniting the charge.
How"famous last words"ard