Professor Cohen was very confident and talktive. He gave us a brief introduction about his group at UC Berkeley. He had six students at present, four Ph. D and two Post Doc. He was very friendly to his students. He controlled student's number less than seven since he began his teacher career. If the students had exceeded seven, he did not think he could direct their researches carefully.
He told us a joke in America. After a teacher said "Good morning" in a class, the postgraduate students replied "Good morning". The undergraduate students, however, said nothing and busily wrote "Good morning" in their notebooks.