You can learn many things from children. How much patience you have, for instance.
—Franklin P. Adams
I laughed when I read that quote but one of the best ways to really learn something is to teach it to someone else. So, if you want to learn something really well, teach someone else about it. You will have to take extra time and research to make sure that you understand the subject well enough to explain it several ways. You need to be prepared to answer questions and clarify what you are saying so that your pupils understand.
I am doing this over at my camera blog, Low End DSLR. If you are interested in learning more about some different kinds of lenses, you can take a look at my ongoing series on Classifying Lenses, beginning with the Crop Factor.