This is a video of Mark Rober discussing the physics of fluidized beds, "a physical phenomenon occurring when a quantity of a solid particulate substance is placed under appropriate conditions to cause a solid/fluid mixture to behave as a fluid." In this case, sand and air make the mixture act like a fluid (previously: the same phenomenon demonstrated on a much smaller scale). Science -- isn't it amazing? Although perhaps even more amazing is the deal I'm about to get on that warranty-voided hot tub.
Keep going for the video, but he does some small scale examples first, so skip to 3:10 if you don't feel like learning and just want to see the hot tub hijinx, then again to 7:20.
Thanks to TFM, who prefers hot tubs filled with the traditional scalding hot water.