Who is listening? What do they hear?

This past summer, at a large international scientific meeting where every contributed talk was allowed 20 minutes, I wandered into a session that seemed intriguing but dealt with a topic about which I knew nothing. After a few hours, I had heard several incomprehensible talks, a couple that justified my intrigue, and one from a fellow who spent 15 of his 20 minutes enumerating the things that he would not include in his talk. Some months earlier, I had given a colloquium in a physics department where I had a number of friends. My talk was a flop; I carried on about many things that interested me but not them. The following week, for another colloquium at a different university, I used the same title but gave a completely reworked talk, and it was very well received. All of which raised for me the following question: What really makes...