Once upon a time is how all stories should begin.  "Once upon" sets up the unforgettable thought that this story only happened once, is unique, and therefore must be told.  But where is it?  Why must it be told?  Who is telling it?  Who needs to hear it?  All of those questions will be answered in someway.  "A time" tells us that it has fixed point in history.  We can remember the moments, or we read about it somewhere in a history book in a high school class long ago.  But when did it happen?  Why is it important?  Have I heard it before?  Will it teach me something?  No doubt everything we read will teach someone something.  Once could argue that anything and everything that can happen  and that people do, will teach someone something.  The question is, is it a lesson worth learning?