There once was a maths/compsci lecturer who was in the middle of a lecture on some obscure proof or other. At some point in the proceedings, he declares an assertion as "obvious", whereupon a student calls out "is it?" The lecturer pauses for a minute scratching his head, then starts to pace back and forth in front of the students. After five minutes or so, he walks straight out of the lecture hall, leaving a bunch of reasonably amused students. Ten minutes later, he bursts back into the lecture hall clutching a few sheets of written paper, declares to the class "Yes, it is obvious", and carries on with the lecture.