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Velcro, coloured scanning electron micrograph (SEM). Velcro is a two-sheet material that is used as a reversible fastener on clothing and fabrics. One sheet consists of hooks (green), the other of loops (yellow). When the two sheets are pressed together, the hooks attach to the loops, giving a secure grip but one that can be easily undone. (KEYSTONE/SCIENCE PHOTO LIBRARY/JUERGEN BERGER/SCIENCE PHOTO LIBRARY)