(When I launched the Advent Calendar of Science Stories series back in December, I had a few things in mind, but wasn't sure I'd get through 24 days. In the end, I had more than enough material, and in fact didn't end up using a few of my original ideas. So I'll do a few additional posts, on an occasional basis, to use up a bit more of the leftover bits from Eureka: Discovering Your Inner Scientist...)