Why do stars twinkle?

A long time ago in school I was told that stars twinkle because they are constantly exploding - but our Sun was "different"...hmmm...anyway, an often asked question by children and the answer is because of the Earth's atmosphere. Anyway, years later (many years later), today actually, I came across this excellent explanation and a stunning photograph of Sirius from the superb (and occasionally mentioned) Bad Astronomy blog by Phil Plait:

 Siriusly twinkling

If you live nearly anywhere on Earth — those of you north of 73° you’re out of luck, but I’m guessing there aren’t many of you! — and look to the southeast shortly after sunset, you’ll see the figure of Orion. Follow the three belt stars to the east, and you’ll see a bright star: Sirius, the brightest star in the night sky. If it’s near the horizon, you may see it twinkling madly: flickering, dancing, perhaps even changing colour.

So, next time you're asked, now you know a great place to go to answer the question.