I use Feline Fresh (pine litter) and it clumps. Clay litter is dangerous because it can cause blockages if ingested .. in both cats and dogs. It also has dust which can lead to respiratory infections. However, feline fresh has no dust, smells nicer than clay, is flushable, holds odor better and clumps really well too. I have always used clumpable cat litter.. if it doesn't urine sits in the bottom and :x Scoopable Feline Fresh: Scoopable Pine Cat Litter / Environmentally Friendly Cat Litter It lasts longer as well, doesn't get so scummy as quickly as clay does.