From the Ri Channel's View the Tales from the Prep Room series, this is how you make a fluidized bed of sand: Making Sand Swim. Watch how these solids suddenly behave a lot like a fluid as air escapes between the sand particles, causing them to float. It’s definitely one of the more strange and fascinating demonstrations that we’ve seen.
Related watching: ScienceDemo.org, and at RIGB.org, a video list that I put together for The Royal Institution that has our favorite STEM videos for kids… well, at least some of our favorites. There are so many.