Sea otters feast on tough-shelled prey like clams, mussels, crabs, and sea urchins. Their powerful jaws and specialized molars help them crush some food, but for the hardest shells, they need a little extra help. That’s where their special pockets and their favorite rocks come in.⁠ ⁠ Under each forearm, sea otters have flaps of loose skin that function like built-in pockets. These pouches allow them to store extra food while diving, preventing the need to return to the surface too often. But what really sets these pockets apart is that many sea otters keep a special rock tucked away in them—a tool they use over and over again. Yep, otters have a favorite rock.⁠ ⁠ Unlike other animals that use tools occasionally, sea otters show remarkable consistency. Once an otter finds a rock that works well, it often keeps the same one for months or even years, storing it in its pocket when not in use. This behavior is not perfectly understood, but it suggests not just intelligence, but also a sense...