I've been teaching long enough to know that doing derivations in class is equivalent to pouring ice water on the proceedings, so I do this sparingly.

我已经教了这么长时间书,所以我知道在课堂上讲推导,就好像泼冰水一样，所以我在讲这个的时候很小心。

麻省理工公开课 - 固态化学导论课程节选