In a High School physics course the teacher brought in an electric motor. The wiring of the motor had been modified so that the stator winding circuit could be opened. This configuration is usually carefully avoided; the stator winding is supposed to be powered whenever the rotor windings are. The purpose of the demonstration was to explain why this rule is important.

The motor was turned on in its normal mode and ran quietly with no load at some constant rate. Then the professor opened the switch gingerly and briefly. The motor made a very alarming sound while the stator circuit was open. The rate of rotation rose rapidly while the switch was open. It was open less than one second.

I think that most students were alarmed. The professor appeared alarmed but not surprised; that was not his nature. I realized at that point that I did not know why the stator circuit was important and I now had a visceral understanding that it was. I then wanted to know why where before I had not been particularly curious.