Ask HN: What's your favorite physics demonstration?

I asked this 12 hours ago, but I thought I'd ask once more, 1/2 a day later, to try to catch a different audience ... So let me ask - what's your favorite physics demonstration? Specifically ballistics, but feel free to mention others. Personally I like the hunter and the monkey, showing deviation from a straight line exactly matches distance fallen in a given time under gravity. I also like the cannon firing vertically on a (uniformly) moving wagon and returning to land in the cannon. But what's yours?