the physics I've learned by listening to my students

A long time ago I wrote a note to myself:  write a blog post about the physics I've learned by listening to my students.  Unfortunately, now I don't remember what specific physics content I was thinking about or any concrete examples.  I just know that many times through the semester, listening to my students talk about physics revealed to me not only the ways they though about physics, but also the ways I thought about physics. 

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.