Of all the books they get at the library, Martin and Sylvia love best the Digbe and Firble series about two mice who are best friends.  Brother and sister love to act out all of the Digbe and Firble stories: Martin plays Digbe and Sylvia plays Firble.  When the children complain that they are tired of playing out the same books over and over, Momma suggests they make up their own story!