20．Pick up a pencil．Lift your foot．Raise your arm．These directions may sound simple，but many parts of your body must work together for you to make those movements．Your joints are parts of your body that help your bones move．With little effort，joints move bones in different directions．But some people's joints do not work well because of illness or injury．Their joints sometimes have to be replaced with artificial ones．
Joints look simple．You might think that scientists would have no trouble making artificial joints that work as well as real joints．After over a century of trying，scientists are just beginning to find ways of making artificial joints that work well．
What problems have to be solved in building an artificial joint？First，some joints have to be strong enough to support most of a person's weight．The first artificial hip-joint，built in 1891，failed because it could not hold up a person's weight．
Second，joints have to be flexible to allow movement in different directions．
Third，joints must be made of materials that the body will not reject．Usually，the body reacts to foreign substances by producing cells that attack and destroy the substances．To the body an artificial joint is a foreign substance that should be destroyed．
Scientists have finally found ways to solve these problems．Nowadays the artificial knee-joint is made of a special metal and plastic．The metal is a combination of silicon （硅） and stainless steel．The metal is strong，so it can support the weight of the body．The plastic is a flexible material．It allows the joint to move easily．Medicines have been discovered that will prevent the body from rejecting the artificial joint．People who have lost the use of a joint can get an artificial joint that works almost as well as the original one．
81．Which parts of our body help our bones move？（No more than 2 words）The joints
82．How many years have scientists been working to find ways of making artificial joints that work well？（No more than 4 words）Over a century
83．How does our body react to foreign substances？（No more than 9 words）It produces cells that attack andd estroy the substances
84．Why can the special metal and plastic used for artificial joints？（No more than 10 words）Because the metal is strong and the plastic is flexible．
分析 本文讲述了科学家研制出假关节，以来帮助那些关节受伤的人能自由移动，以及在制作假关节时需要注意的事情．
解答 81．The joints．/Our joints．细节理解题．Your joints are parts of your body that help your bones move这句话告诉我们人体关节部位帮助人身体移动．可以直接找到答案．
82．Over a century．/More than 100 years．细节理解题．在第二段After over a century of trying，scientists are just beginning to find ways of making artificial joints that work well．这句话里的时间状语over a century，说明科学家开始开始研制的时间．
83．It produces cells that attack and destroy the substances．/By producing cells that attack and destroy the substances．细节概括退．在注意事项的第三点里，Usually，the body reacts to foreign substances by producing cells that attack and destroy the substances．这句话告诉我们身体对外来物质的抵抗主要因为这些物质会产生一些有攻击性的细胞．
84．Because the metal is strong and the plastic is flexible．细节概括题．最后一段The metal is strong，so it can support the weight of the body．The plastic is a flexible material．可以得知，金属坚硬，塑料柔软，是作为人造关节的首选材料．
点评 阅读表达，要求学生在阅读理解的基础上，根据短文后的题目，在文中找到相关的内容，并用简洁、准确的句子，将理解的内容，书面表达出来．做题时，注意表达的内容不要超出规定的字数．
