Shapeshifting is a common theme in mythology, folklore, and fairy tales. It is also found in science fiction literature, children's books. In its broadest sense, shapeshifting occurs when a being (usually human) either has the ability to change its shape into that of another person, creature, or other entity or finds its shape involuntarily changed by someone else. If the shape change is voluntary, its cause may be an act of will, a magic word or magic words, a potion, or a magic object. If the change is involuntary, its cause may be a curse or spell, a wizard's or magician's or fairy's help, a deity's will, a temporal change such as a full moon or nightfall, love, or death. The transformation may or may not be purposeful.