Number 150 (Story #2), November 5, 1993 by Phillip F. Schewe and Ben Stein HINGED NETWORK CRYSTALS , a hypothetical class of materials, have bizarre mechanical and thermal properties: when stretched they become thicker (i.e., they have a negative Poisson's ratio) and more dense; when heated they contract. These materials, investigated by Ray Baughman of Allied Signal, Inc. (Morristown, NJ) and Douglas Galvao of the Physics Institute in Sao Paulo, Brazil, consist of twisted chains of interconnected polydiacetylene molecules. Unlike other negative-Poisson materials (generically called auxetics), these computer-designed materials derive their predicted novel properties from the interconnectivity of various parallel and non- parallel bonds in alternating layers, which allows the chains to twist, hingelike and with a minimum amount of energy expenditure, without changing the network bond lengths. Such crystals, if they could be synthesized by chemists, might also have interesting optical and electrical properties. (Nature, 21 Oct.)