Answer: Sea Otters
Tool use in the animal kingdom is a relatively rare thing, and even among tool users, sea otters stand out for something most of us can certainly relate to—they have favorite tools and they like to keep them handy.
Specifically, sea otters use rocks to crack open shells and a good rock is not to be wasted. Underneath the arms of each sea otter, at the axilla, they have flaps of skin where they store food as they gather it. But they also keep a sturdy rock with them for future meals by storing it in the same underarm flaps of skin. Once they’ve scooped up enough clams or other treats to work on, they’ll float on their back or swim to a nearby outcropping and set to work using their rock to bust open the shells.
Lest you think that’s the only endearingly anthropomorphic thing sea otters do, we have a bonus bit of trivia for you. Sea otters sleep while floating on their backs and will hold paws with each other, like a long chain of children on a field trip, in order to avoid drifting apart while sleeping. These groups, known as “rafts”, are segregated by sex and generally range in size from ten to one hundred otters.