A one-molecule heat engine that converts information into work — the thermodynamics of measurement
Leo Szilard (1929) showed that a demon who measures which half of a box a single molecule occupies can extract exactly kT ln 2 of work per cycle by inserting a piston and letting the gas expand isothermally.
But the demon must erase its memory to complete the cycle — and Landauer's principle says that erasure costs at least kT ln 2 of heat, exactly canceling the gain. Information has thermodynamic cost.