My brothers and I engineered a really powerful spud gun. We were looking to fire the potatoes across a river, so we went to the cops to make sure we wouldn't get into trouble. At first they were concerned, but when they found out it was powered by air pressure and not hair spray they said it was OK. Little did they know that the air pressure one is more than 10 times more powerful than the combustion ones. We never did manage to get the potatoes across the river, but it was fun nonetheless.
http://www.youtube.com/watch?v=yL_-1d9OSdk
http://lanseybrothers.blogspot.com/2007/02/how-old-are-you-now.html