Why does the sun set?

Another great family story (sorry Mike) is the time Emma Jane asked why the sun sets.

Emma Jane was almost 4 years old and we were driving down a busy Tulsa street and the sun was setting in front of us.  I was on the phone (shocker) and Mike was driving through rush hour traffic. 

Emma:  Daddy, why does the sun set?
Mike:  It just does.
Emma:  But why?
Mike:  (who is worrying about lane closures, rush hour traffic and the setting sun in his eyes) Because.  It just does.  Every day it sets.  It just does.

(Mommy hangs up the phone.)
Emma: Mommy, why does the sun set?
Me:  Well, that is a good question.  The sun doesn't actually go anywhere.  It is a fixed ball of gas in space but the earth is spinning and our side of the earth is spinning away from the sun so it looks like it is setting but we are really just turning away from the sun.  And tomorrow morning we will spin around and face the sun again and it will seem like the sun is rising but it is really just our side of the earth spinning back around to face the sun again.
Emma:  Oh.  Could you tell daddy that, because he doesn't know.

Mike looks over at me and mouths the word "thanks".