I briefly flirted with the idea of becoming a teacher, I took some courses in education and one of those was how to teach elementary math. I figured it would be easy, shit was almost as complicated as algebra because they had us learn all these sort of things (like mbl's picture) and all sorts of 'tricks' to help students figure out how to do basic math. Looking at it as an adult who already knows how to do basic math, it seemed overly complicated but if you think back to when you were first learning, the visual aids can help children understand the concept. In particular, children that are visual or tactile learners. Of course, it is not for everyone, just like learning straight from the book's written or a teacher's verbal explanation is not going to get through to all the students.