Bill has been in different stories, a doorknob, an ice cube tray, and a tiger without stripes to give you some sense of the wild directions these stories take. Of course some stories turn out better than others, but it is truly wondrous how the kids have started adding bits and pieces of detail to the plot and character. Pretty soon they begin to develop a sense of confidence, and, just as critical, the sense of power that comes with creating a story. For a 12 year old, I would plumb his interests and get him writing about something that fascinates him. The key point with metaphors and similes is originality.
Originality is the critical challenge.
Just as you hope to deliver story twists that are unexpected, coming up with metaphors and similes with your own creative stamp on them adds to the fun. Of course devising similes and metaphors that are at the same time vivid, true and original, is difficult work. That's why so many are trite and UNoriginal. And that's why I often create them from observations I happen upon in day to day life, and write them down in a notebook, with the remote chance that the opportunity may come along to actually use them. My daughter who is 14 is a professional storyteller.