Biomolecular machines are complex open systems par excellence. Many of them turn the free energy of hydrolysis of ATP into useful functions, like shortening muscle, advancing a transcription bubble along DNA, and pumping ions across membranes. Yet how can free energy decreases get turned into anything useful in an unconscious thermal bath of biomolecules? It is proposed that a significant contribution to the power stroke of myosin and some conformation changes in other biomolecules is the osmotic pressure of a single molecule (e.g. a phosphate ion) expanding a trap. Necessary conditions to achieve this efficiently are given, and the elements of a mathematical justification. It is proposed as a design principle for nanobiotechnology. Joint work with D.J.C.MacKay.