Two legs on three legs, eating no legs, four legs runs away with no legs, so two legs throws three legs at four legs. Four legs runs back with no legs. What are they?

A man on a stool eats a fish, a cat steals the fish, the man throws the stool at the cat, which brings back the fish. A man on a stool eats a fish, a cat steals the fis

Source: Riotous Riddles