What happens when you ignite hair spray in an enclosed space? It produces a lot of pressure. Focus that pressure on a common garden potato, and you suddenly have a high velocity potato. Since people always ask, I'm not sure exactly how far it will shoot - but it does throw a largish potato several hundred yards at an impressive velocity.
Shown here are two intrepid potato gunners (Alan and Steve) loading the potato gun. Note its attractive PVC construction with duct tape accenting. The builder of this particular model (Uncle Keith) is keeping an eye on the proceedings.
More details: The gun is built from 1.5" (barrel) and 3" (chamber) PVC pipe. Two cleanout plugs are present on the rear ends of the Y joint. One has a gas grill igniter embedded in it, which serves as the trigger. The other is removed to place the propellent in the chamber, then replaced for firing. The propellent shown here is Rave hairspray ("Extra Hold #2" - note that four of the first five ingredients listed are alcohol, butane, propane, and something else that is flammable. "Glade" air freshener has also been proven to work). Before the propellent is added, a potato is rammed down the barrel with a broomstick. Firing occurs by triggering the igniter, and, assuming a proper hair spray-air mixture is present, the potato is then launched. Apples and moldy apricots also work, with various degrees of messiness. Other objects, e.g. tomatoes, have been tried, but don't work very well.
Here the potato gun is about to be fired. Note that since the trigger is so far to the rear of the gun, two people are needed to aim and fire accurately.
I have built my own potato gun, similar to the one pictured here except that the Y-joint is reversed so that the "trigger" points in the muzzle direction. This allows a single person to aim and fire with accuracy. It also has a slightly larger combustion chamber, which gives it a bit more oomph. Boom!