No one answered until Mary stood up and declared: "You should not be asking us a question like that. I'm going to tell my parents, and they will go and tell the headteacher who will then fire you!"
In the best traditions, the teacher ignored her and asked the question again. Little Mary's mouth fell open, as she said confidently to those around her: "Boy, is she going to get in big trouble."
The teacher continued to ignore her and made a final plea to the class. Eventually, 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."
The teacher purred: "Very good, Billy." She 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; and three, one day you are going to be very, very disappointed."