A doctor was once teaching a class of medical students at a famous hospital in Edinburge. An injured man was brought in, and the doctor turned to one of his students and asked him, “What’s wrong with this man ?” “I don’t know, sir,” the student answered. “Shall I examine him and find out?” “ There’s no need to examine him ,” said the doctor. “You should know without asking questions. He has hurt his right knee. Didn’t you notice the way he walked ? He hurt it by burning it in the fire. You see his trousers leg is burnt away at the knee. This is Monday morning. Yesterday was fine, but on Saturday the roads were wet and muddy ( 泥泞的 ) . The man’s trousers are muddy all over. The man fell down on Saturday night.” The doctor then turned to the man and said, “You had your pay on Saturday and went to a public house and drank too much. You got muddy and wet on the way home. Because you had drunk too much, you fell on the fire and burnt your knee. Is that right? “Yes, sir,” said the man.