Here's a very simple cardboard box air cannon video tutorial. You can use it to shoot down light targets or fill it with smoke and blow smoke rings. Basically tape a cardboard box closed and cot a round hole in one end. Push on either side to make a gust of air out of the cannon. This makes a great science experiment about air pressure. The video shows using a fog machine to fill the cannon with smoke. You can skip that step. I would substitute using dry ice and water to make smoke or burn some incense. The video is in Japanese, but that doesn't matter. A picture is worth a thousand words.
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