You are hereLlyn Ebyr
Edwin Sidney Hartland gives the following tale about how a mother retrieved her twin children after they had been replaced by fairy changelings in his ‘The Science of Fairy Tales’ (1891)’ ‘Water's antagonism to witches is notorious; and ample use was made of it in the old witch trials. It is equally obnoxious to fairies and their congeners. In a Welsh story from Radnorshire, when the mother has been by the egg-shell device convinced of the exchange, of her own twin children, she takes the goblin twins and flings them into Llyn Ebyr; but their true kinsmen clad in blue trousers (their usual garb) save them, and the mother receives her own again. In other tales she drops the twins into the river; but in one case the witch who has been credited with the change bathes the child at a mountain spout, or pistyll, and exacts a promise from the mother to duck him in cold water every morning for three months. It is not very surprising to learn that "at the end of that time there was no finer infant in the Cwm."