The human arm and a crocodile's tail are both important for mobility, but they serve different functions. The human arm is able to make a wide range of movements, such as gripping and leveraging objects, thanks to its flexibility and the presence of joints. On the other hand, a crocodile's tail is mainly used for swimming and providing stability in the water, as it is composed of strong, rigid scales. It also helps them to maneuver quickly through the water by providing thrust and leverage.