Even though shellfish are unclean and not fit to eat (Leviticus 11:10), they are useful in developing scratchproof coatings. A new mixture using molecules made from chitosan, derived from the shells of crabs and other crustaceans, self-heals with exposure to ultraviolet sunlight after being scratched. The process takes about 30 minutes, a significant improvement over previous self-healing substances ("Coatings That 'Self-Heal' in Sun," News.BBC.co.uk, March 12, 2009).