"What Gets Bigger and Bigger?" 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."