The otters usually carry a favorite rock in their paws or in a pouch under their forearm. They use it to smash shells open. This makes them one of the few animals that use tools. They grow to 1.0 to 1.5 m (3.3 to 4.9 ft) in length and weigh 30 kg (66 lb). Although once near extinction, they have begun to spread again, from small populations in California and Alaska. Sea otters are one of the smallest mammals in the ocean.