  Mr. Perkins, the biology instructor at a posh suburban girl's junior 
  college, said during class, "Miss Smythe, would you please name the
  organ of the human body that expands to six times its normal size, and
  define the conditions."
  
  Miss Smythe gasped, then said freezingly, "Mr. Perkins, I don't think 
  that is a proper question to ask me.  I assure you my parents
  will hear of this." With that she sat down red-faced.
  
  Unperturbed, Mr. Perkins called on Mr. Johnson and asked the same 
  question.
  
  Mr. Johnson, with composure, replied, "The pupil of the eye, in dim
  light."
  
  "Correct," said Mr. Perkins. "And now, Miss Smythe, I have three 
  things to say to you.  One, you have not studied your lesson.  Two,
  you have a dirty mind. And three, you will some day be faced with a
  dreadful disappointment."
