To Learn, You'll have to Teach

I just finished a book called "The Witch of Portobello" by Paulo Coelho. The one thing that struck me the most is the idea that in order to learn, you'll have to teach something that you are not familiar with.

Gonk was doing a tutorial and commenting that he has to learn the tutorials first before he could go into his class because he did not know anything about it. In the end he realised it was not that difficult to begin with. He commented that if he knew what he knows now, uni would have been a breeze.

How true it is that in order to truly learn you really have to teach something that you do not know.