In mythology, folklore and fantasy fiction, shapeshifting, or metamorphosis is the ability of an entity to physically transform into another being or form. This is usually achieved through an inherent faculty of a mythological creature, divine intervention, or the use of magic spells or talismans. The idea of shapeshifting has been present since antiquity and may be common in all cultures. It is present in the oldest forms of totemism and shamanism, as well as the oldest extant literature and epic poems, including works such as the Epic of Gilgamesh and the Iliad, where the shapeshifting is usually induced by the act of a deity. The idea persisted through the Middle Ages, where the agency causing shapeshifting is usually a sorcerer or witch, and into the modern period. It remains a common trope in modern fantasy, children's literature, and works of popular culture. The most common form of shapeshifting myths is that of therianthropy, which is the transformation of a human being into an animal or conversely, of an animal into human form. Legends allow for transformations into plants and objects, and the assumption of another human countenance (e.g. fair to ugly).