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There are enough signs of the apocalypse already: the global financial crisis, reports that one in four mammals are at risk of extinction, the Cubs (briefly) making the playoffs. So maybe it's no surprise that a huge milestone (or tombstone perhaps) slipped by without much notice. The national debt broke $10 trillion on Sept. 30, but honestly there was so much going on that we can forgive everyone for being distracted. Including us. Ten trillion is an almost unimaginable number -- so colossal that the even the people who worry about debt had trouble anticipating it. The National Debt Clock in Times Square, for example, didn't even have room for that many digits. On Sept. 30, they had to squeeze the "1" and the dollar sign into the same box. How much is a trillion dollars anyway? Like we all learned in school, it's a thousand billions, and as the old line goes, "a billion here and a billion there and pretty soon you're talking about real money." But t...