“Listen, I’ve taken three classes with Professor ____, you do not want to try a be a brown noser. He unrelentingly called on me once he figured out my name. If I could give you any advice, it would be to just sit in the back and keep your head down!”
The above speaker establishes ethos, by establishing his familiarity with a specific professor. Through this particular student’s “3 classes” taken with this professor, the student can now speak about his experiences to others. After taking three classes, others can safely assume that this student is quite familiar with this professor. By first establishing his first-hand experiences about being “unrelentingly called on”, the student’s argument about being discrete in the professor’s class appear credible. Therefore we trust this student’s advice to other students who might take this professor’s class.