Micro-electromechanical systems (MEMS) not only can think like Pentium chips but also can sense the world and act upon it. Fashioned largely from silicon with techniques adapted from the microchip industry, they are fast and generally cheap to mass-produce. And they boast startling mechanical sophistication--gears, levers, motors, and mazes--in packages no bigger than a standard computer chip: Scientists have already moved MEMS into various stages of conception and development for making laboratories on chips, data-storage technologies, cell-manipulating gadgets, propulsion systems for microsatellites, locking mechanisms for nuclear weapons, and many other applications.