"If there's one thing Ph.D. has taught me, it's when to label
something 'obvious'. If it requires less than an hour of thought,
it's 'obvious'." -- Breno, Ph.D. candidate in general relativity (and
violinist, not that that's relevant) as
quoted by
silmaril
Well, that explains this old story...
The professor said "Oh, it's obvious," and turned back to his work. Then he stopped for a minute, looked back at the board, and started writing equations, deriving a path from 57 to 58. The students struggled to keep up as he manipulated the math.
Half an hour later, the professor finally turned around again. "Yes, obvious," and resumed the lecture.