One cold day an elder lady sent by her doctor. When he came,she told him that her left leg was very sore(疼痛) at the knee,the pain was sometime bad enough to make walking difficulty. She asked him what the trouble could be as she had never before suffered any similar discomfort. After making a careful examination,a doctor said:"You are at a very good state of health for a woman of your age. I think trouble in your right knee is just as a matter of old age. We get diseases as we get old. The trouble will almost certainly clear up in spring. " " Nonsense,doctorl" she said," My left knee is perfect,and it is exactly the same age as my right. "