I spent several class periods teaching this skill. We used base ten blocks to discover it, I've used all the things I can think of, and now as I grade a few errant papers, I notice that they still don't get it. I thought most had it, so I moved on to a new concept. Now I see that many more don't have it than do. I'm looking for any and all ideas! BTW, this is a 4th grade spec ed class, so the more concrete & visual, the better. I'm teaching the same material as the regular math teacher, but spending much more time on each concept.