Today in History of Math I talked about Leonhard Euler. (I didn't finish. He was a busy fellow...) After I described the Law of Quadratic Reciprocity (which he discovered, but did not prove) and explained that number theorists consider it one of the most beautiful theorems in mathematics, one student asked, "What is it good for?"
That, of course, touched off Canned Rant No. 4B, on beauty versus utility. After a few sentences I was in full swing: "Is the Mona Lisa useful? Is Chartres Cathedral useful?" From the back of the room: "Some people think so." I stumbled, paused, and said, "Touché... And me a Catholic, too. I should have caught that."
I let the rant go. No way I was going to recover my head of steam after that.