Richard Feynman (my father) divided perpetual motion machines into Perpetual Motion Machines of the First Kind, which violate the first law of thermodynamics (AKA conservation of energy), Perpetual Motion Machines of the Second Kind, which violate the second law of thermodynamics (entropy must increase), and Those Other Perpetual Motion Machines, which purport to get their energy from other laws of physics.    The Second Kind is rarer; the only example I'm familiar with is one that my dad showed me the plans for. Once in a while, someone would come to him with a perpetual motion machine prospectus and ask if it was a good investment. One of these was a machine that claimed to extract heat energy from the air and run a generator with it. They had built half the machine, which worked as far as they could test it, and were looking for gullible investors to pay for finishing it. By carefully going over the immensely complex plans, my father and I found a "heat exchanger" that was supposed to take warm freon and cool air, and produce hot freon and cold air. Naturally, this heat exchanger was in the as-yet unbuilt portion.  Those Other Perpetual Motion Machines get their energy from heretofore unknown physics, like cold fusion, vacuum fluctuations, or secret new physical laws that will be disclosed only on the payment of $1,000,000. These are worthy of much more serious physical investigation, since it is entirely concievable that someone will discover a new easy way of making energy.  Back around 1970, my father went to a public demonstration of one of Those Other Perpetual Motion Machines. He discovered an electric cord running out of the back of the machine, plugged into the wall. When he unplugged the machine and pointed out to the inventor that a perpetual motion machine that had to be plugged in wasn't really perpetual, the inventor pushed a button on the control panel and the machine exploded. Several spectators were severely injured; I believe one man lost an arm. The inventor sued my father on the grounds that he had caused the explosion; my father suspected that the explosion was deliberate. The trial ended up with my father not having to pay. Epistemic status: Childhood recollections, untainted by fact-checking. Originally an email from 1997.