This video shows closeups of how sodium polyacrylate, which is used in fake snow, diapers and things like max-oads, behaves when water is added. Apparently you can dry out the sodium polycrylate and reuse it later.
Sodium polyacrylate is a polymer that has the ability to absorb as much as 200 to 300 times its mass in water. It is used in gardening as well as fake snow, and diapers. This is several clips of small, 0.5mm - 1mm granules absorbing water.