This demonstration video shows you how to make a potato gun/cannon for fun. For this you'll need a 4" pipe, a 2" pipe, a 2" to 4" pipe converter, a 4" screw on end cap, a spark battery, a screw-on narrower and PVC cement. Glue on the parts together as shown. Insert a potato at the 2" end and add a propellant to the 4" end and screw on the end cap. Ignite the propellant as shown to fire the potato.