I read this joke as a kid. It must be ancient, but it's still funny. A high-school biology teacher asks Miss Smith, "What organ of the body enlarges to ten times its normal size during periods of excitement?"
Miss Smith stammers and blushes and looks at the floor and says, "I don't want to answer that question."
The teachers turns to Mr. Jones and asks the same question.
"The pupil of the eye," answers Mr. Jones.
"That's correct," says the teacher.
Then the teacher turns to Miss Smith and says, "Two things are obvious, Miss Smith. The first is that you didn't study your lesson last night. The second is that your wedding night is going to be a terrible disappointment to you."