well said: However, not all machines that 'run forever' fall under the category of perpetual motion. We can give examples of possible ways which can move indefinitely, without breaking any of these two laws. For example, it is possible to design a machine to run on the differences in either barometric pressure, tidal, gravity or temperature shifts between night and day. Such machines have a source of energy, and even if they can do work indefinitely, that is, as long as both machine and the universe exist, they do not fall under the category of perpetual machines, and hence are theoretically possible. The problems arise, when such perpetual machines use a kind of energy which has not yet been yet identified and accepted in mainstream science, sometimes, even if there is undeniable proof of its existence. Classical examples of such systems include cold fusion, magnetic, and gravity or zero point energy converters. However, even in such cases, these cannot be categorised as perpetual systems, and do not break the first and second laws of thermodynamics. All they break is usually a paradigm, and for the same reason, we might be able to show, that the above wheel pictures may in fact not be mythical wheels, but real wheels which turn using energy residing in the vacuum, and that after all, do not violate neither the law of conservation of energy, nor the known laws of thermodynamics. Philip