Azobenzene, a polymer molecule, has a unique characteristic: It changes from one shape to another when exposed to light of different wavelengths. Scientists led by Hermann Gaub, a professor of physics at the University of Munich, have used this quality to build a tiny gadget. They attached the molecule to a glass plate on one end and to a miniscule silicon spring on the other (see graphic). Then they exposed the azobenzene to alternate wavelengths of light. As the molecule bent and straightened, it pulled on the spring and then released it, rather like a firing piston.