&For this demonstration a small gas cylinder was suspended under a wire which was stretched across the lab.
The piece of wood I was holding in my left hand had a nail sticking out, behind the cylinder. When I hit the cylinder with the hammer it would make a hole allowing the gas to escape rapidly, sending it shooting across the room at around 20meters per second.
With a little bit of maths you can use this to illustrate Newton's second law of motion, F=ma, and explain how rockets work.
It looks as if I hurt myself, but like many things this was a bit of theater to make it more memorable for the students.
It was shot by a student during a year 10 class during my first ever term as a teacher, I didn't know at this point that it was almost a capital crime. I can't publish his name, but he went to study A-Level physics with me a few years later and got quite a good grade :-)