I came across the joke while cleaning up my e-mail box, it was sent by my friend Hakan who is pursuing his PhD in mathematics. I would like to share it
A father who is very much concerned about his son's bad grades in math decides to register him at a catholic school. After his first term there, the son brings home his report card: He's getting "A"s in math.
The father is, of course, pleased, but wants to know: "Why are your math grades suddenly so good?"
"You know", the son explains, "when I walked into the classroom the first day, and I saw that guy on the wall nailed to a plus sign, I knew one thing: This place means business!"