There's a common variation without the student. The prof just pauses after saying "obvious", puts down the chalk, stares at the blackboard, runs out and then comes back, says "it is indeed obvious" and continues the lecture.
There's a common variation without the student. The prof just pauses after saying "obvious", puts down the chalk, stares at the blackboard, runs out and then comes back, says "it is indeed obvious" and continues the lecture.