And one of the finest stanzas of all time:
I took an awful non-Euclidean geometry class my last semester as an undergraduate. The professor mumbled incomprehensibly, and sometimes stopped lecturing to play his flute. There were assigned textbooks, but no reference was ever made to them. He would listen to questions, but then answer with something completely unrelated. About the only way to make sense of the problem sets was to figure out where he'd gotten them from and read that. He disapproved of us doing that, though, so we started using "anyone want to study Lobachevsky after class?" as code for "I know where this problem set comes from, meet me in the library and I'll share."