Perpetuum Mobile

Throughout the ages, many people have been fascinated (read: obsessed) with the idea that it might be possible to somehow construct a device which would provide enough power to keep itself going as well as do some sort of work. Nobody, however, was able to demonstrate a successful working device to the satisfaction of the Real Scientists, which led to the formation of one of the fundamental Laws of Thermodynamics: You can't get something for nothing. This enabled the Real Scientists to simply wave their hands and go "Pish and tush!" whenever anyone wanted to demonstrate such a machine for them, thus saving them valuable time that they could then use for Real Research (such as what would be the most efficient way to cut down an entire forest).

( It just keeps going and going and going... )