One cold day, an aged lady was sent for her doctor. When he came, she tells him that her right leg was very sore(酸痛)in the knee. The pain sometimes was bad enough to make it walking difficult. She asked him how the trouble could be because she had never before suffered some similar discomfort. After a careful examination, the doctor said, “You are in a very bad state of health for a woman of your age. I think the trouble in your right knee is just matter of old age. We get diseases as we get old. The trouble will clean up on spring!” “Nonsense, doctor!” she said, “My left knee is perfect, and it is exactly same age as my right.”