I write this while I prepare for school. It's 5 am, and I've finished breakfast and I'm having my usual cup of coffee. Yesterday, I was able to rewrite two solutions to the problems assigned in the particle physics course I'm enrolled in, but procrastinated again when it came to marking my students' papers. I'll do that later, in between classes.
Part of the fun of teaching is rethinking what you know, and hopefully understanding the thing you're teaching better. I have friends who teach some subjects to force themselves to study the things they didn't learn as undergraduates or graduate students.
This means making mistakes sometimes in class, or at least experiencing hang time. You know how it is when the professor pauses to think about what he has to say because there is a conflict of ideas in his head. Sometimes, the professor is only a few days ahead of the class-- it happens most often for new assitant professors, because they tend to get the teaching load garbage.
Garbage doesn't mean worthless topics; in fact, the teaching garbage loads are actually the hardest topics to teach; they tend to be demanding mathematically. The problem sets are hard to mark as well because the professor has to read the papers carefully, and make sure of correctness of reasonning. Examples include graduate core courses like electromagnetism, quantum mechanics, and statistical physics. The prerequisites for these courses include almost all the mathematical methods we are supposed to learn as undergraduates.
This means a lot of preparation. I think a good rule of thumb is 8 to 10 pages of carefully reasoned lecture notes. You'll also need to prepare what your eventual boardwork should look like. Also included in the preparation is a syllabus (this can only be prepared after actually teaching the subject), and the syllabus ought to include a list of learning objectives, reading assignments for each day of class, and suggested problems for solution.
I've taken on the challenge of preparing for teaching a solid state physics course even though I know almost nothing about it. I won't be teaching it this semester; maybe a few years down the line, I will offer a seminar for the theory group based on the materials I and a friend will prepare. It's better this way compared to an actual teaching load because I really want to learn this material, and I do not elarn things fast. I usually spend a lot of time thinking and solving problems to make sure that what I know is self-consistent.
I agreed to learn solid state physics for the following reason: The things that I do know well (relativity) are not as relevant to most experimentally-minded physicists I know compared to solid state physics. Solid state physics also has the advantage that there are experiments ongoing here as opposed to, say, astrophysics or general relativity.
I write this on my new netbook; and I'm getting to know the keyboard. I didn't understand what a chiclet keyboard was while the saleman talked about it. After some thought, and a google search, I realized that the keys looked like gum made by Cadbury Adams. I got that from a wikipedia entry.