cat litter
compound.a special material, like sand or small stones, that you put in a box for a pet cat to use as a toilet.
compound.a granular, absorbent substance used to line a tray for a domestic cat's waste. Usually uncountable in its mass sense, though 'litters' may refer to different brands or types.
“I need to buy more cat litter at the supermarket.”
“The apartment started to smell because he forgot to change the cat litter for three days.”
“Advances in biodegradable cat litter have led to products made from recycled paper and wood shavings rather than traditional clay.”
usually functions as an uncountable mass noun.
use this to teach the difference between 'litter' as trash and 'litter' as a specialized material; it is also a good example of a noun-noun compound where the first noun acts as a modifier.