It is hard to teach an introductory science course. You have to go back to basics and make sure that what you cover all the fundamental principles and concepts and that ain't easy. That's why the best teachers in introductory courses are often senior professors and lecturers with plenty of experience behind them. They have learned what's important and what's not and they can tell the difference between wheat and chaff.
Teaching is so easy, anyone can do it!].
One of the first things you learn when you start teaching is that you have to know the content inside and out — it’s simply not enough to know the bare minimum that you expect the students to master, because as a teacher, you need to push just a bit farther to get them up there. You need to be able to lead them to knowledge, and you need to be able to point off in the distance to all the cool stuff they can learn if they continue. How can you inspire if you’re not drinking deeply from the Pierian Spring yourself?Keep this in mind next time we discuss teaching evolution and biochemistry. Teachers have to be experts and it takes a lot of work to make sure you know your content. If what you're teaching is not correct then you are not a good teacher no matter what the student evaluations say. And it's not only a question of accuracy—as PZ points out, you need to be more than a few steps ahead of your students in order to inspire them to do better.