That above statement can be said two ways. In one way it can be said that it's important to teach another, so that they can learn what you know. It's legitimate, but it's not really what I mean by the title. What I'm really saying is "teach another... so you can find out what you really know."
I've been considering this quite a bit this past week, since I made an agreement with one of my assistant scout leaders to teach some of his younger children how to play guitar and sing, respectively. It's one thing to know how to do these things. It's quite another to teach them to others, and especially to others who really don't understand what you are telling them, or why it is important.
When we get into the habit of teaching, often we think that it's going to be to someone who wants to learn, and who already knows why they want to learn what you are going to teach. For some, though, they may not have the full picture, or it may not be as glamorous at first. As I explain to my friend's son, he's probably going to be more frustrated than excited the first few weeks he practices, because his left hand fingers are going to get very sore, and his right hand is going to get very tired, and all of this for notes that will likely not ring out or sound like much of anything at first. His daughter is, I know, already wondering why I'm having her practice yawning and holding her palms against the small of her back as she does it. What's the point in all of this?
the point, as I explained to both of them, is that they have to build a foundation to go farther and actually perform with their instruments, if that's what they want to do. For a guitar player, that comes from getting sore fingers until they build calluses; for a singer, it means practicing yawning until they can feel how to do proper back breathing to support their voice. It means doing some constant repetition, some consistent warm ups and working through the pain and the frustration so that they can get to something fun. Likewise, we can't keep them waiting for the fun too long. Yes, they have to get through the very basics first, but then we have to reward them with something; a song, a favorite melody, or some net trick that will give them a "wow" moment, and make them more eager to learn going forward.
Sometimes in testing, when we teach others, I think we forget the "wow" moments, that point where we can help someone discover something really interesting, or see something out of the ordinary. We need to make sure we don't lose sight of those wow moments, and remember what enthuses us might not enthuse others... but it just might. So reach out and teach others. You may surprise yourself at what they learn, and what you learn in the process as you discover what you really know.