Why do cats purr, and why do they seem to be the only ones doing it?

To the attention of teachers and parents . We join the international initiative #KidsAskRanger (#ЗапитайПриродознавця).
Children ask questions about nature, their native land, who lives and what grows, how people affect the environment and how the environment affects us. Then we answer, play and draw.
Send your questions - audio / video, handwritten text or drawing to [e-mail] marked "KidAskRanger". Be sure to indicate the child's name, age and hometown .