The word kangaroo comes from the language of the Aborigines, the natives of Australia. It means "I don't understand" and was the answer to the question of the British navigator James Cook what kind of animal it is. What a misunderstanding. In their hind legs the marsupials have a kind of elastic band. It makes them jump up with every jump. The faster a kangaroo jumps, the further the animal is catapulted forward and the less energy it consumes.