The title of this post was more than long enough already, but it could probably be clarified a little with one more phrase: If You Can’t Teach It, You Don’t Know It Nearly as Well as You Could.
This is something I figured out back when I was teaching first- and second-year college physics: I never knew my subject as well as when I was able to successfully convey it to someone else.
I’ve come to think that teaching a concept is an important stage in learning it. It’s not that everyone must be a teacher, though it is a valuable and important skill. It’s not that you can’t successfully use a concept you haven’t taught – because of course you can. Successful teaching, though, involves expressing a concept clearly and succinctly, and often from multiple directions. Being forced into that level of clarity and circumspection has the effect of solidifying a concept for the teacher as well as the student.
I believe in this idea enough that I’ve used it to choose topics for conference talks or lunch-and-learn sessions. I’ll be working on something, I’ll need to use a certain tool, and I’ll notice that I don’t know nearly enough about the tool – so I’ll commit to a talk and create a situation where I must then learn enough about the tool to teach someone else to use it, and to answer the kinds of questions I anticipate will be asked (roughly 50% of which are questions I also have). It doesn’t make me an expert off the bat, but it provides a solid foundation that, combined with repeated use, will get me there.