Sodium polyacrylate is one of the most used 'super-absorbants'. You can find it in loads of products where it's used to control moisture, daipers and certain soil agents are a good example. You can harvest it from some products for significantly cheaper than buying it as 'fake snow', which surprisingly is still cheaper than buying from a chemical store such as Aldrich, but they probably sell a much higher grade.
Anyone interested in the chemistry:
Since sodium polyacrylate (Na(pa)) is a polymer, it can exhibit qualities of its small monomer units whilst also having properties of a larger single molecule. Specific to Na(pa) is it can form hydrogen bonds and ionic bridges between unconnected monomers. It can also bond with moisture in the air, which creates an oxide layer on the outside surface, similar to what is observed in solid metals. This is why the dry polymer is a powder with an inert surface rather than a solid.
When there is significant water pressure to penetrate the outside layer water will enter the powder grains. The water breaks both the hydrogen and the ionic bonds between chains, creates an ionic solution. The water can form hydrogen bond bridges between the chains, but these are significantly longer than the ionic or single hydrogen bonds, so the Na(pa) expands a lot. This allows free waters to sit between the bonds, giving the polyacrylate its liquid character. Na+ ions will be dissolved in the free water solution.
As with my 'why wouldn't two pieces of wood placed on top of each other become one', I didn't do much research on this. I'm fairly certain I'm right though (I'm a degree chemist).