Source:

    Matt Calhoun (https://math.stackexchange.com/users/1417/matt-calhoun),

    Anecdotes about famous mathematicians or physicists, 

    URL (version: 2011-03-25): https://math.stackexchange.com/q/28912

HOWEVER, this is disputed, see https://hsm.stackexchange.com/questions/7247/in-a-popular-anecdote-who-took-20-minutes-to-decide-that-a-thing-was-obvious

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    My mechanics professor (G. Horton) took lectures from Pauli. Pauli
    apparently had a very rapid and hectic lecturing style; he would just
    turn his back to the class and start talking and writing on the board.
    So one day a student interrupts and says "excuse me professor but I'm
    having trouble following the step you did back there..." to which Pauli
    immediately replies, "oh, it's obvious!"; but the student not being
    satisfied with this explanation began to elaborate on the specific
    issue he was having. Pauli listened thoughtfully as the student tried
    to explain his question. Afterwords, he didn't answer right away;
    instead he paced back and forth for a bit, went into the hallway and a
    little while later he came back in and said:

    "Ok, I've thought about it, and it is obvious!"

    and then he went right back into his lecture where he left off.