Which human body part increases to 10 times its size when stimulated?

Found this lovely read from it occured to me:

The 6th grade science teacher, Mrs. Parks, asked her class, "Which human body part increases to 10 times its size when stimulated?"

No one answered until little Mary stood up, angry, and said, "You should not be asking 6th graders a question like that! I'm going to tell my parents, and they will go and tell the principal, who will then fire you!" With a sneer on her face, she then sat back down.

Mrs. Parks ignored her and asked the question again, "Which body part increases to 10 times its size when stimulated?" Little Mary's mouth fell open; then she said to those around her, "Boy, is she gonna get in big trouble!"

The teacher continued to ignore her and said to the class, "Anybody?" Finally, Billy stood up, looked around nervously, and said, "The body part that increases 10 times its size when stimulated is the pupil of the eye."

Mrs. Parks said, "Very good, Billy," then turned to Mary and continued, "As for you, young lady, I have three things to say:

One, you have a dirty mind, two, you didn't read your homework assignment, and three, one day you are going to be very, very disappointed."