Seeing Red Means Danger Ahead The color red often means danger and by paying attention, accidents can be prevented. In the future, the color red also may help (51) danger at construction sites. Thanks to new work by engineers, bridge supports or (52) kinds of materials could one day contain a color switching material. It will turn red (53) a structure collapses or falls apart. The secret (54) the color switching material is a particular type of molecule. A molecule is a group of atoms held together by chemical bonds. Molecules come (55) all shapes and sizes, and make up (56) . You can see, touch Or feel. How a molecule behaves depends on (57) kinds of atoms it contains, and how they’re held together. When a polymer (聚合物) containing a color switching molecule called a mechanophore (机械响应性聚合物) which is about to break, it produce a (58) When a polymer with mechanophore molecules becomes "injured" or (59) , one of the mechanophore bonds (60) and the material turns red. "It’s a really simple detection method." says Nancy Sottos, one of the scientists who worked on the project. Sottos and her team tested the color switching polymers in their lab. The test (61) proved encouraging. There is a way to get rid of the color light when a bright light is shone on the mechanophore. The broken bond is fixed and the red color (62) . This "self-healing" may be a problem for engineers. They need to use the color changer in big construction projects that will be (63) in sunlight. And sunlight will make the mechanophore’s warming system (64) . Sottos and her fellow scientists still have (65) work to do before the color switching molecules can be used outside the lab.