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Seeing red Means Danger Ahead The color red often means dangerand by paying attention, 1 can be prevented. At railroad crossings, flashing red lights warn cars to stay back. A red light at a traffic intersection tells cars to stop, so 2 dont run into other cars. In the future, the color red also may help prevent danger 3 construction sites. Thanks to new work by engineers, bridge supportsor other kinds of materialscould one day contain a color-changing material. It will turn red 4 a structure collapses or falls 5 . A tiny molecule may make a big difference in future warning systems. A polymer 6 a color-changing molecule called a mechanophore turns red seconds before it snaps. The technology may one day allow damage to materials or structures4 to be easily 7 . The secret behind the color-changing material is a particular type of molecule. A molecule is a group of atoms held together by 8 bonds. Molecules come in all shapes and sizes, and make up 9 you can see, touch or feel. How a molecule behaves depends on what kinds of atoms it contains, and how theyre held together. When a polymer containing a color-changing molecule called a mechanophore is about to breaks, it produces a color. When a polymer with mechanophore molecules becomes injured or 10 , one of the mechanophore bonds breaks and the material turns red. Its a really simple detection method, says Nancy Sottos, one of the scientists who worked on the project. Were 11 up this one bond, and it changes color.Sottos and her team tested the color-changing polymers in their lab. The test 12 proved encouraging. | 
