What part of the body increases 10 times when excited?

A biology professor was addressing his class, wanting to see if they'd
read the assigned text. He asked Miss Smith to stand. She does.

Professor: Miss Smith, what part of the human body increases ten times
when excited?

Miss Smith blushes and hesitates and giggles.

Proessor: Miss Smith, please sit down. Miss Jones, please stand and
tell me if you know what part of the human body increases ten times when
excited.

Miss Jones: Yes, Professor. It's the pupil of the eye.

Professor: Very good. Thank you Miss Jones, you may sit down. Miss
Smith, will you please stand again. (She does) I have three things to
say to you.
1. You have not done your homework
2. You have a very dirty mind.
3. You're in for a big disappointment.