In European folklore and folk belief, 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, an animated piece of wood that would soon appear to die. Other claimed changelings would be identified by their wizened appearance, voracious appetite, malicious temper, inability to move, and other unpleasant traits.[1] Medieval chronicles record instances of this, which is one of the oldest known pieces of folklore about fairies.[2] The motivation for this conduct stems from the desire to have a human servant, the love of a human child, or from malice.[3] Some people believed that trolls would take unbaptized children. Beauty in children and young women, particularly blond hair, attracted the fairies.[4] In Scottish folklore, the children might be replacments for fairy children in the tithe to Hell[5]; this is best known from the ballad of Tam Lin.[6] Some folklorists, who held that the fairies were memories of inhabitants of various regions in Europe who had been driven into hiding by invaders, held that changelings had actually occurred; the hiding people would exchange their own, sickly children, for the healthy children of the invaders.[7] The reality behind many changeling legends was often the birth of deformed or retarded children. Among the diseases with symptoms that match the description of changelings in various legends are spina bifida, cystic fibrosis, PKU, progeria, homocystinuria, William's syndrome, Hurler's syndrome, Hunter's syndrome, and cerebral palsy. The greater proneness of boys to birth defect correlates to the belief that boy babies were more likely to be taken.[8] 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.[9]