Our DS5(bday today!) loves stories too. DH and I lucked out by having my mom, who is a very willing and imaginative story-teller, living nearby and being the first culprit for DS. His story obsession started when he was playing tower defense games, and the stories are all about the creatures in the game. (This has evolved to including creatures of various different made-up planets, etc, etc, depending on what books we are reading with him.) It's more of a joint effort, with DS5 interrupting to make sure the story is exactly how he wants it, and grammy making him take a turn every so often. But it's definitely an ongoing saga, which would have filled pages of books by now! Whenever DS sees his grammy, he says "Get on with the Tower Story!" DH and I used trickery to get out of this, by telling feeble stories when it was our turn to tell the story, or killing off main characters, or saying "The End." (The characters we killed off of course would be resurrected by DS and grammy, and DH and i were off the hook!)
