[S3 Physics: Density] A solid has a definite shape and volume, as is the case with sand. However, it's possible to change the behaviour of sand by inserting pressurised air underneath it, causing it to act like a fluid.
The Particulate Model of Matter depicts solids as objects with a fixed structure. While this is true for sand and many other examples of solids, modifying its density (via introducing a sufficiently large amount of air underneath) divides the overall density between that of sand and air, resulting in the sand behaving like fluid particles millions of times larger than liquid molecules.
Watch it in action here: