"Biology" joke

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, which under the appropriate conditions, 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 Miss Johnson
and asked the same question. Miss 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."