专题限时训练(三十八) [科普知识型阅读理解(二)]
(限时：25分钟)
(一)
a huge cloud of dust rose and an overpass disappeared. this was the scene in zhuzhou, hunan province on may 17.nine people were killed and sixteen injured.
things were similar in 2008 in the sichuan earthquake. thousands of people were buried in the ruins and lost their lives. what if we could have warned them?
people are always racking_their_brains to find a way of preventing buildings' collapse. better materials and technologies help, but they are not a solution.
just like humans, a building has its own life cycle from“birth” to “death”．if we know when a building is going to collapse, we can repair it in advance or get out of it before it falls.
now, scientists at the university of illinois have developed a material that turns red before it breaks. the invention could be used in things like climbing ropes or bridge supports.
the research was led by nancy sottos, a professor at the university's beckman institute, and douglas davis, a graduate research assistant.
the secret behind the colour-changing material is a type of molecule(分子)．a molecule is a group of atoms held together by chemical bonds. imagine you and your friends stand in a circle, holding hands. each person stands for one atom; your hands represent the bonds，and the entire circle represents a molecule. if one person lets go of his or her hands, the molecule changes colour.
the research team put the molecule into a soft material. when the researchers stretched the material, it turned bright red for a few seconds before it broke into two pieces. when they repeatedly stretched and relaxed the material，without breaking it, it only turned a little red.
the major problem is that light can get rid of the red colour. when the team shone bright light on the molecule, the broken bond was fixed, and the colour disappeared.
if the bright light keeps the red colour from appearing, the material's warning system will be useless. scientists still have a lot of work to do before the colour-changing molecules can be used outside the lab.
1．what does the passage mainly introduce?
