A sea otter has a loose fold of skin under each forearm, a pocket, and in it, as often as not, a rock. It dives for a clam, comes up, rolls onto its back, sets the rock on its chest and knocks the shell against it until it gives. Then it doesn’t throw the rock away. A good one gets kept, carried down on the next dive and up again, sometimes for a long while. Nobody is quite sure why that rock and not the thousand others on the seabed. The best guess I have is the ordinary one: it is easier to keep the thing that already works than to go looking for a better one, and after enough clams it isn’t really a rock any more. It’s the rock.