At the Men-an-Tol there is supposed to be a guardian fairy or pixy who can make miraculous cures. And my mother knew of an actual case in which a changeling was put through the stone in order to get the real child back. It seems that evil pixies changed children, and that the pixy at the Men-an-Tol being good, could, in opposition, undo their work.
‘The Fairy Faith in Celtic Countries’ p179, W Y Evans Wentz, 1911.