As I've been spending time on my upcoming presentations on jQuery, I've found that I'm learning a lot. This isn't anything new that I've noticed however. People can be very knowledgeable in a particular field, and you (yes, you who is reading this) may be very good at something, but would you consider yourself an expert on the subject?
There's always more to learn about a discipline, technology or field. When you try to teach or inform others, you'll find that you only gain more knowledge on the topic.