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Shown to the left is a trash can with ~6 in diameter hole in the bottom. A bungee cord holds a rubber mat over the other end. I slip the black strap over my neck so the trash can hangs conveniently in front of me. By slapping the rubber mat, donut shaped vortices (smoke rings) are driven out of the can. Since we generally cannot use smoke to make the vortices visible inside, I have students hold the thin plastic sheet while I shoot vortices at it. The sheet moves like a small invisible ball is thrown into it. Because the vortices are very stable you can back away from the sheet ~30 feet, if you like. There is the bang when you hit the can, but some time passes before the sheet moves. Clearly this motion of air is not the sound wave. Grade school children had a great time taking turns shooting invisible donuts across the classroom.


If I want to explore the nature of scientific explanations, this is what I use. I explain that I loaded the can with a few demons before class and one jumps out each time I hit the can. Of course, no one believes me, but I can come up with all sort of ad hoc fix ups to explain anything ---and therefore I explain nothing! This is bad science since my theory cannot be proven wrong.


I've been told that the Germans worked on air cannons in WWII for the purpose of shooting down aircraft.

Air Cannon