[Isura]

A mathematician and his girlfriend are out to dinner. They are arguing about how smart the average person is. The guy gives the average person credit for being pretty smart. But his GF disagrees. So when she goes to the restroom, he asks the waitress to help prove his point. He aranges to ask her what the integral of xdx is. He tells her to answer x^2/2. So when his GF gets back, he carries out his plan when the waitress comes back. He asks, "what's the integral of xdx". She answer "x^2/2". And when walking away, she turns around and says "plus a constant!".