The spurious story

We look at a story that was apparently inserted after the fact in the Gospel of John: the story of the adulterous woman.  We discover that it is a very complex, multi-faceted story that requires great study.  The Pharisees want to stone the woman to death.  Jesus writes in the dirt and mysteriously, the Pharisees slither away.  Why did they do this?  What are the various explanations for this passage?  Why should we be glad the story did make its way into the Gospel?  And where did the story originate, anyway?