hey, a while ago in high school out teacher was showing uss tuff to do with the transferance of energy and stuff like that, what he brought into class one day was a rubbish bin, with a hole cut out bottom, and on the lid, there was a rubbish bag(the big black ones) , on top with a rubber band taped to it. basicaly the way it worked was, the band is taped to the bag,m when you pull the band back, you pull back the bag also, which pucks in air into the hole in the bottom of the bin,but at this time the hole is actually the top of the cannon, as thats where the air enters and comes out. now when the air is being sucked into the bin, our teacher also had a hose from a fog machine attached to it, which made the bin suck up air, now when he let the band go when the bin was full of air, a ball of air shot out, and the fog remained inside that air ball so we could see where the ball went, which was around 20 meters, anyway, the fog ball could nock over polystyrene cups from 20 meters. i want to know exactly how to build one of these as i cant really remember how they look like as i have forgotton, i was reminded of these once when i saw one being used on rollercaoster, in the minishow, elliot and the surfing scientist.