Today's Joke: A simple biology question

Mr. Jacobs, the biology instructor at a posh suburban girl's junior college, said during class, "Miss Arnold, 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 Arnold gasped, then said freezingly, "Mr. Jacobs, 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. Jacobs called on Miss Jones, another student, and asked the same question. Miss Jones, with composure, replied: 

"That would be the pupil of the eye, under conditions of dim light."
"Correct," said Mr. Jacobs.
"And now, Miss Arnold, 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 someday be faced with a dreadful disappointment."