Dated, but still funny. A high school biology teacher asks the class, "What organ enlarges to ten times its normal size during periods of excitement?"
He calls on Miss Smith, who stuttters and blushes as says, "I don't want to answer that question!"
So he calls on Miss Jones, who answers, "The pupil of the eye."
The teacher says, "Your answer is correct." Then he turns to Miss Smith and says, "Two things are apparent. First, you didn't study your lesson last night, and second, your wedding night is going to be a terrible disappointment to you."