As a child, Hod Lipson lost Lego pieces constantly. Now the 39-year-old director of Cornell University's Computational Synthesis Lab can build replacement parts on the spot. Completed last year, Lipson's fabrication machine, called a "fabber," can print thousands of three-dimensional objects, everything from toy parts to artificial muscles, using dozens of materials, including PlayDoh, peanut butter and silicone, by following simple directions sent to it by a PC.