(I apologize if this is more physics than math.) I attended a talk last night by an MIT professor who is working with "photonic crystals", which have the property that they can selectively allow photons of certain wavelength/energy to propagate, while disallowing other photons. These devices are non-dissipative, so that the non-selected photons simply bounce back. One type of crystal is made of a conductor with small holes, and the size of the holes determines which wavelengths can get through. (Think Faraday cage.) This professor suggests that one can put such a photonic crystal "filter" in front of a "blackbody", and thereby allow only the most energetic photons to leave. _Presumably_, the photons that bounce back into the blackbody exchange energy with the blackbody to reconstitute the classic blackbody radiation spectrum, so that the only photons leaving the blackbody are those with high energies. This is not a perpetual motion machine of the first type, because energy is conserved, but isn't it a perpetual motion machine of the second type? Why doesn't this idea violate the non-existence of Maxwell's demon?