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-changing material．It will turn red【53】______a structure collapses or falls apart． The secret【54】______the color-changing material is a particular type of molecule(分子)．A molecule is a group of atoms(原子)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-changing molecule called a mechanophore(机械响应性聚合物)is about to break，it produces 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-changing polymers in their lab．The test【61】______ proved encouraging． There is a way to get rid of the red 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 warning system【64】______． Sottos and her fellow scientists still have【65】______work to do before the color-changing molecules can be used outside the lab． (51)