This makes me a wee bit nervous because of our experience with statements like this...
Not to drift off-topic, but I appreciate this sub-discussion, because, though no one has used the 'G' word with us, DS6 has for years utterly fallen in love with one isolated topic after another, notably, the alphabet, numbers, and the solar system. Not only his teachers, but even I get worried myself from time to time that he is over-obsessing. I'm coming to the view that they represent his comfort zone and provide a useful portable intellectual structure. We need to nudge him outside of the zone a bit, but it's good that he has it.
As it turns out, the solar system is a terrific launching pad for a whole raft of learning topics, from measurement, to geometry, to chemistry. It's actually a very sensible long-term framework to establish.
