(appeared in "The Kids are Alright" episode)
A changeling is 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, a glamorized piece of wood that would soon appear to grow sick and die. The motivation for this conduct stems from the desire to have a human servant, the love of a human child, or malice. Most often it was thought that faeries exchanged the children, and simple charms, such as an inverted coat, were thought to ward them off.
According to some legends, it is possible to detect changelings, as they are much wiser than human children. When changelings are detected in time, their parents have to take them back. In one tale of the Brothers Grimm, there's an account of how a woman, who suspected that her child had been exchanged, started to brew beer in the hull of an acorn. The changeling uttered: "now I am as old as an oak in the woods but I have never seen beer being brewed in an acorn", then disappeared.
Some people believed that trolls would take unbaptized children. Beauty in human children and young women, particularly blond hair, attracted the fairies. In Scottish folklore, the children might be replacements for fairy children in the tithe to Hell; this is best known from the ballad of Tam Lin. Some folklorists believe that fairies were memories of inhabitants of various regions in Europe who had been driven into hiding by invaders. They held that changelings had actually occurred; the hiding people would exchange their own sickly children for the healthy children of the invaders.