Saunders Mac Lane (1909 – 2005) was an American mathematician who became known for being one of the proponents of category theory. During an interview with The Mathematical Review in 1976, he reminisced about the time when he was learning about geometry in high school:
I recall one occasion involving a theorem about a triangle: I knew that the specific shape of the triangle did not matter and that its vertices could be lettered at will. So when I went to the board, I drew the triangle upside down, changed the letters labeling the vertices, and presented the proof at flank speed, all to the evident distress of my teacher, as some of the students (my friends) egged me on. In retrospect, it is apparent that I understood the proof from my first geometry class, but that I did not at all see how to communicate the proof to my fellow students; I must have been a real nuisance to the young teacher.