Merge pull request #473 from daattali/patch-1

shell intro: add analogy for "cd -" that I found useful with many nov…

_episodes/02-filedir.md

@@ -667,7 +667,8 @@ Run `pwd` and `ls -F` to ensure that we're in the directory we expect.
 > *the previous directory I was in*, which is faster than having to remember,
 > then type, the full path.  This is a *very* efficient way of moving back
 > and forth between directories. The difference between `cd ..` and `cd -` is
-> that the former brings you *up*, while the latter brings you *back*.
+> that the former brings you *up*, while the latter brings you *back*. You can
+> think of it as the *Last Channel* button on a TV remote.
 {: .callout}
 
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