My favorite is when the said annoying person is very obviously not listening to anything that is going on because they're too busy thinking of the next brilliant inspired question to ask, and then *light bulb* they come up with a brilliant question, raise their hand and ask the question that the professor just spent 20 minutes explaining to the person who asked right before the guy who wasn't listening asked it. This has happened in my classes many many times. And usually the professor is way way too nice and just says 'well, as i just said...' and then gives a three minute short form of his earlier 20 minute response
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