When creating a model of any kind, in order for it to encourage real conceptual growth, there must be a multi-step process involved that encourages deconstruction of a concept and reconstruction based on evaluation and an output goal.
Here is a simple variation on a design process. It is important that you find one that works for you and your students. In my experience, the experiment and the revision phase are the most difficult phases for students and they need considerable support in not committing to one idea and staying with it the whole way through.
