Placing his chair on his desk the professor instructs the class, "Using every applicable thing you've learned in this course, prove to me that this chair DOES NOT EXIST."
So, the students get busy writing essays proving that this chair doesn't exist, except for one student.
He spends thirty seconds writing his answer, then turns his final in to the astonishment of his peers.
The day comes when all the students get their final grades...and to the amazment of the class, the student who wrote for thirty seconds gets the highest grade in the class.
His answer to the question: "What chair?