"Biology - six to one" 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."