Dirty Mind

Mr. White, the biology professor, at a posh suburban girl's school, asked during class, "Miss Smith, would you name the organ in the human body, which under the appropriate con-ditions, expands to six times it's normal size, and define the conditions."
Miss Smith gasped, then said snottily, "Mr. White, I don't think that is a proper question to ask me. I assure you that my parents will hear of this." With that, she sat down red-faced.
Unperturbed, Mr. White called on Miss Jones, and asked the same question. Miss Jones, with complete composure replied, "The pupil of the eye, in dim light."
"Correct," said Mr. White. "Now, Miss Smith, I have three things to say to you: one, you have not studied the chapter I assigned. Two, you have a dirty mind. And three, you will someday be faced with a dreadful disappointment."