A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. Fins are also used to increase surface areas for heat transfer purposes, or simply as ornamentation. The fins on the tails of cetaceans, ichthyosaurs, metriorhynchids, mosasaurs, and plesiosaurs are called flukes.