Yeah, we were taught in basically the exact same way -- moving around different colored weights on plastic print-outs of balances. I'll also note that this was a public (non-magnet) school -- a reasonably good public school in the suburbs, to be sure, but not what I would think of as an especially advanced primary education.

I join lots of other commenters as being genuinely surprised that the content of this post is understood so little, even by mathematicians, as it all seemed pretty common sense to me. Indeed, my instinctive response to the first meditation was almost exactly what Eliezer went on to say, but I kept trying to think of something else for a while because it seemed too obvious.

I love this idea, so I've taken it to the next level: http://sphotos-a.xx.fbcdn.net/hphotos-ash4/405144_10151268024944598_356596037_n.jpg

Hanger, paper clips, dental floss, tupperware, pencil, ruler, and lamp. If we're trying to be concrete about this, no need to do it only part way.