A doctor was teaching some medical students at a famous hospital. A patient in, and the doctor asked one of his students, "What's wrong with the man?"

   "I don't know, sir," the student answered." I need to examine him."

   "There's no need him," said the doctor." You should know without asking questions. He  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. It  Monday morning now. Yesterday was fine, but on Saturday the roads were wet and muddy(泥泞的).  The man fell down on Saturday night."

    The doctor then turned to the man," You had your pay on Saturday and went to a public house and  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.