Make a tennis ball launcher
Just like a potato cannon, this monster tennis ball launcher works in the same fashion. Using PVC piping and sprinkler valves you can launch a tennis ball off into the distance. It\'s a fairly simple build when all the parts and instruction are listed. There\'s even a link to modifying the sprinkler valve with pictures to guide you. This is an air-powered launcher unlike most of the combustion ones out there. It’s more consistent because you can regulate the amount of air you use. Just make sure you use it responsibly, they can be very dangerous.
>> http://sky.prohosting.com/jchou/launchers/tb1-inst.htm