A changeling is a creature found in Western European folklore and folk religion. It is typically described as being the offspring of a fairy, troll, elf or other legendary creature that has been secretly left in the place of a human child. The apparent changeling could also be a stock, an enchanted piece of wood that would soon appear to grow sick and die. The theme of the swapped child is common among medieval literature and reflects concern over infants afflicted by as then unknown diseases, disorders, or mental retardation.
Some fairies love beautiful human babies and will steal them and leave an old, near-death fairy in their place. Humans have been known to place iron bars on or in their baby cradles to protect them from such thefts.
A human child might be taken due to many factors: to act as a servant, the love of a human child, or malice. Most often it was thought that fairies exchanged the children. Some Norwegian tales tell that the change was made to prevent inbreeding: to give trolls and humans new blood, humans were given children with enormous strength as a reward. In some rare cases, the very elderly of the Fairy people would be exchanged in the place of a human baby, and then the old fairy could live in comfort, being coddled by its human parents. Simple charms, such as an inverted coat or open iron scissors left where the child sleeps, were thought to ward them off; other measures included a constant watch over the child.