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 exchange for a human child. The motivation for this conduct stems from the desire to have a human servant, the love of a human child, or from malice. Some people believed that trolls would take unbaptized children. The reality behind many changeling legends was often the birth of deformed or retarded children. Most often it was thought that faeries exchanged the children, and simple charms, such as an inversed coat, were thought to ward them off.
Source: http://en.wikipedia.org/wiki/Changelings
