It's stuff you add to regular non clumping litter to make it clump. I did a search on it, but obviously the word littermate comes up a lot here.
Supposedly it saves money and it clumps tight enough so you don't have to dump the litter aside from scooping out the 'little treasures'.
Supposedly it saves money and it clumps tight enough so you don't have to dump the litter aside from scooping out the 'little treasures'.