One of the joys of teaching any subject is watching the "aha moment" when a new concept or piece of information slips almost visibly into place. One assumes a synapse fired, a neural pathway was created (or completed), and the mysterious process of learning just occurred. It's one of the best experiences I know. And the mystery is that the only things worth really knowing haven't changed--ever. Sure, the details change, the names change, the packaging shifts, but the truly important things don't change.
The challenge for teachers is to honor the ancient, understand the new, and help bridge the gap for the students. As much as I love this process, some days I am just not up to the challenge. There are not enough hours in the day, days in the year, or years in my life to know enough to be a truly competent teacher. It's humbling.