I'll be the first to admit I've been slow to join this bandwagon. I just didn't understand what all the fuss was about. And then I tried some mentor sentences with my students. Holy cow! What great opportunities I'd been missing!
I've always been fascinated by the beginning of books. How do authors hook you in? What do they do to make you read beyond the first couple of sentences? What makes a book hard to put down? I'm sure you, like me, have heard kids say, "It needs to grab me right away or else I won't read it."
How could they get that kind of great start to their own writing?
Show them what the experts are doing. Take sentences out of the book they're reading and let them play with them. We started the year reading Fish in a Tree. The first two sentences from that book ("It's always there. Like the ground underneath my feet.") gave them the impetus to go in a variety of different directions!
Take apart the sentence so they can see how it's constructed. Let students identify the nouns, verbs, adjectives, and adverbs in the sentence. Have them discuss the tone or mood. What does it make them think of? That Fish in a Tree beginning felt ominous to a lot of my students. What was "always there?" There were some excellent discussions about how to create that kind of mood. Even the students who didn't want to create that kind of mood themselves, felt they learned something.
Scaffold the sentences. Then set them free! Each time we looked at a sentence, students had two opportunities to write: one where they filled in the blanks of the same sentence and one where they created their own completely. To be honest, after the first lesson, I think I could have done away with the scaffolded version. Most students were ready to start imitating and creating on their own.
If you've not tried this, please do! And enjoy writing with your students!