Thinking back on those lectures, they were a bit on the dry side and as there was a lot of restlessness during the lectures. Also the paper planes had started

Thinking back on those lectures, they were a bit on the dry side and as there was a lot of restlessness during the lectures. Also the paper planes had started to appear in earlier lectures and the prof did pretty much ignore them. I think it in this instance it was the plane crashing into the blackboard (and making a pretty loud thud) only inches from his left ear that finally made him snap.