A philosophy professor asked his students just one question for their final exam. The question was :-

“How are you going to make me believe that this chair in front of you is invisible?”

It took most of the students an hour to finish writing their answers, all except for one, well known for being exceptionally lazy, who only took five seconds. 

When the results were posted. The lazy student got the highest score. His answer was :-

“What chair?”