Far back in the depths of time, the then Dean of the Faculty (Theo Kalikow) came to observe me teach one of my classes, University Physics.
During the observation a student asked me to do one of the problems at the end of the chapter. It involved adding waves (interference). But while the chapter derived a formula that could be applied to adding waves of the same amplitude, this problem had two waves of different amplitude. It thus had to be done from scratch.
I got stuck (rare). In front of the Dean. At that moment a student (one of my best ever) simply said, “Take a derivative.” CLICK. We finished the problem.
This was much more impressive than if I had not been stuck in the first place.