In Western European folklore, particularly Irish folklore, there are legends of babies - and sometimes even adults - stolen off to the Faerie Realm while leaving a faerie in their place. The most common term for these creatures is Changelings. They are also known by other Western European countries as Mamuna, Wechselbald, or Boginki. There were many theories regarding why this would happen. The humans were destined to be servants, as payment for the faeries blood dues with the Devil, crossbreeding, or out of malice. They may be taken because of faeries wanting a strong, healthy baby instead of a sickly one, or for elderly fairies looking for comfort in their last years. Sometimes, the child would be replaced by a block of wood or wax replicating the child, which would appear to grow sick, followed by the child's death.
Changelings were often suspected if a child began to appear weak, feeble, and sickly. Increasing signs included thin, bony limbs, growing uglier in appearance and behavior, and dark eyes wise beyond their years. If a child ate well yet remained malnourished, experienced developmental delays, or had any physical deformities, these were also considered signs of a switch.
One way to surprise a changeling to reveal itself was by tricking it into speech or making it laugh. You could also use herbs, salves, and seeds, leave eggshells by the fireplace, mistreat it, whip it, or put it in the oven. Most parents believed that once the changeling revealed itself, their child would be returned. In contrast, some believed if they were nurtured and loved to become healthy and happy, the faerie parents would desire them again and switch them back.
To protect a child from being switched, you could have the child baptized, keep the fire lit in their room before baptism/christening, tie a red ribbon around the baby's wrist, have it wear a red hat, or placing an iron, a knife, or scissors above the cradle.