Sixth-grade science teacher Mr. Barnes asks his class: "Who can tell me which organ of the human body expands to 10 times its usual size when stimulated?"
Nobody raises a hand, so he calls on a student. "Mary, can you tell me which organ of the human body expands to 10 times its usual size when stimulated?"
Mary stands up, blushing furiously. "How dare you ask such a question?" she says. "I'm going to complain to my parents, who will complain to the principal, who will have you fired!"
Mr. Barnes is shocked by Mary's reaction, but undaunted. He asks the class the question again, and this time Sam raises his hand.
"Yes, Sam?" says Mr. Barnes. "Sir, the correct answer is the iris of the human eye." "Very good, Sam. Thank you."
Mr. Barnes then turns to Mary and says, "Mary, I have three things to tell you: first, it's clear that you have not done your homework. Second, you have a dirty mind. And third, I fear one day you are going to be sadly disappointed."