Sea otters are known for using tools, but not all of them do. A new study finds those that use tools have better survival odds in a world where their traditional food sources are changing.

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.

Some otters rely on tools to bust open hard-shelled prey items like snails, and a new study suggests this tool use is helping them to survive as their favorite, easier-to-eat foods disappear.