Transitioning to Argumentative Writing with a claim and evidence to support the claim is challenging. To get students to start seeking and citing evidence, I use fairy tales. I ask the questions at the end of the story. When a student replies, I ask "How do you know?"
The students soon start looking to the story to back up their claim.
This process can be used with any short piece of writing as an introduction to argumentative writing; however, I find that the students get particularly passionate when discussing their familiar fairy tale characters.