"Proof of nonexistent chair" joke
An eccentric physics professor is well known throughout campus for having strange tests which often border on the philosophical.
An ill-prepared student goes in for his final exam with this professor, racking his brain to keep all his formulas straight. He sits down, and the professor walks in to start the exam. Grinning, he sets a chair on his desk and writes the exam's only question on the board: "Prove that this chair does not exist." The student groans and drops his pencil, realizing that he hasn't any clue how to solve this problem. Deciding that if he's going to fail, he'll do so with style, the student writes two words on his paper, turns it in, and gets the highest grade in the class.
His essay read simply, "What chair?"
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