Here’s W.B. Yeats’ introduction to the section on changelings in his collection Fairy and Folk Tales of Ireland:
“Sometimes the fairies fancy mortals and carry them away into their own country, leaving instead some sickly fairy child, or a log of wood so bewitched that it seems to be a mortal pining away, and dying, and being buried.
“Most commonly they steal children. If you ‘look over’ a child,’ that is, look on it with envy, the fairies have it in their power.”
[But how can you tell? I mean maybe my kid just kind of shriveled up ’cause he was sick. And babies look an awful lot alike.]
“Many things can be done to find out if a child’s a changeling, but there is one infallible thing — lay it on the fire with this formula, ‘Burn, burn, burn — if of the devil, burn; but if of God and the saints, be safe from harm.'”
[Don’t forget that last part! I know what you’re thinking. But we have to be more open-minded about faith-based medical procedures. And Yeats notes that this incantation is from Lady Wilde, who tended to be a lot gloomier than all the other Irish writers on the subject.]
Then Yeats tells a beautiful little story in just one paragraph, really a thing of beauty:
“It is on record that once, when a mother was leaning over a wizened changeling, the latch lifted and a fairy came in, carrying home the wholesome stolen baby. ‘It was the others who stole him,’ she said. As for her, she wanted her own child.”