As a university professor for much of his life, Dijkstra saw teaching not just as a required activity but as a serious research endeavour. His approach to teaching was unconventional. His lecturing style has been described as idiosyncratic. When lecturing, the long pauses between sentences have often been attributed to the fact that English is not Dijkstra's first language. However the pauses also served as a way for him to think on his feet and he was regarded as a quick and deep thinker while engaged in the act of lecturing. His courses for students in Austin had little to do with computer science but they dealt with the presentation of mathematical proofs. At the beginning of each semester he would take a photo of each of the students, in order to memorize their names. He never followed a textbook, with the possible exception of his own while it was under preparation. When lecturing, he would write proofs in chalk on a blackboard rather than using overhead foils. He invited the students to suggest ideas, which he then explored, or refused to explore because they violated some of his tenets. He assigned challenging homework problems, and would study his students' solutions thoroughly. He conducted his final examinations orally, over a whole week. Each student was examined in Dijkstra's office or home, and an exam lasted several hours.