Also in the better kinds of cat litter. This is not a value judgement (I once bitterly offended the editor of a local "freesheet" by remarking, in all innocence, that his paper was just right for wrapping used cat litter prior to disposal. He took it the wrong way, and never spoke to me again; which couldn't have worked out better if I'd planned it) but a mere observation.

Powdered chalk would probably work too, but I wouldn't advise it.