The chair

Here is something nice I read today:

An eccentric philosophy professor gave a one question final exam after a semester dealing with a
broad array of topics. The class was already seated and ready to go when the professor picked up his chair, plopped it on his desk and wrote on the board: "Using everything we have learned this semester, prove that this chair does not exist." Fingers flew, erasers erased, notebooks were filled in furious fashion. Some students wrote over 30 pages in one hour attempting to refute the existence of the chair. One member of the class however, was up and finished in less than a minute. Weeks later when the grades were posted, the rest of the group wondered how he could have
gotten an A when he had barely written anything at all. His answer consisted of two words: "Which chair?"