A man came to a psychiatrist and claimed to be dead. No matter what argument the psychiatrist used, the patient refused to believe he is alive. Finally, the psychiatrist decided to try indirect proof. No, of course not. The psychiatrist found a needle, pricked the patient's finger, and squeezed out a drop of blood.
Dead men do bleed.
Make sure to ask it a few more times. Any time the professor writes down an approximation on the board you work it out on the calculator and announce loudly that a mistake has been made. Everyone is Welcome.