<!-- m --><a class="postlink" href="http://www.youtube.com/watch?v=jV_9EXhWrGo">http://www.youtube.com/watch?v=jV_9EXhWrGo</a><!-- m -->
Simple principle really. Carbid + water creates a gas, which slowly fills the cannon. Some of the gas will expand out of the cannon through a hole in the back, igniting this stream of gas results in an explosion which propels whatever you want to propel out of the cannon. But the best part is that, if done properly, you can create bangs louder than most illegal fireworks, while this is perfectly legal. :> Spend a day doing that.
Happy newyear and best wishes everyone!