From time to time they would say something so clear, illuminating some relationship between concepts that inside my brain all I could say was "duh." On further reflection, I realized that these relationship were things that I had always taken for granted, not necessarily as blindingly obvious, but I never really thought about explicitly stating it either. Perhaps because, until I heard that student say it, I never fully understood how I myself understood that particular bit of physics. Now I know to listen for that concept in my students' discussions and probe for different types of understanding.
I feel terrible that I can't remember an example of when this happened, but does anyone else have an example of something similar? When have you had your own thinking clarified by a student's question or explanation?
a lightbulb. because that's what turned on in my head.