Mark Rober (previously at Neatorama) builds a contraption in his backyard to play with sand. It's more than a sandbox- it's a hot tub full of sand, which can be made liquid with the injection of air. But you better keep that air flowing, because without it, you're trapped!
Watch Rober try all kinds of weird tricks to show how sand and air acts like water. Oh yeah, if you're going to dive into liquid sand headfirst, you might want to pinch your nose shut. It's still sand. He eventually gets around to explaining the science of how it works, but most of the video is the fun stuff. -via Digg