Sea otters carry a favourite rock around to smash open their food

Floating on their backs, sea otters balance a rock on their chest as an anvil and hammer shellfish against it until the shell cracks. Some otters keep and reuse the very same rock for extended periods, tucking it into a loose pouch of skin under a forearm between dives — a rare case of an animal keeping a personal tool.

— Standard marine biology accounts (Riedman & Estes), Tool Use by Sea Otters — U.S. Fish and Wildlife Service Biological Report, 1990