My friend came over a while ago and we played with my pneumatic cannon. I built the cannon from PVC pipe, a valve stem, and a ball valve. Basically, fill it up with compressed air, shove something down the barrel (normally water balloons) and then turn the ball valve. SHVOOM! Out it goes, really far. Well, we decided to shoot something other than water balloons. So, we dug around in my house and found a hedgehog toy that I got in about 1st grade.
PERFECT! So, we soaked it with water, shoved it down the barrel, cranked it up to 80 PSI and then fired. That thing soared! Everybody should build a pneumatic cannon! Party!