I always used to tell myself "You can never be sure you've really understood something until you try to recreate it". It started from my university days when I noticed that stuff I read and thought "yeah I got this" later turned out to be anything but, when I had to do it without the textbook at hand. Since then, I've always kept the habit. There's literally a pen and notepad two inches to the right of my keyboard. There is also a terminal open always, where I can do this:

  $ learned
    > about Walder's law: the bulk of discussion on programming language 
    > design centers on syntax (which, for purposes of the argument, is 
    > considered a solved problem), as opposed to semantics.

That's an actual example from two days ago. "learned" is just a shell script:

  #!/bin/bash
  read text
  echo -e "\n$text" >> ~/Dropbox/writing/notes/learned.txt

As you can see it auto syncs to dropbox.