"Or is she an elfish spirit, who, as the legends of our childhood taught us, is forbidden to cross a running stream?"
Tam O'Shanter escapes the witch (c.1790)
Public DomainTam O'Shanter escapes the witch (c.1790) - Credit: G. Cook

It is a truism of folklore that, because running water is pure and holy, fairies, demons, witches and their ilk may not cross it. Robert Burns alludes to this in Tam O'Shanter, a riotous poem which tells the tale of the drunken Tam's encounter with a witches' Sabbath in deliciously salty language. When the narrator urges our hero's horse onwards to the bridge where he might escape the hellish mob that pursues him, he reminds him, “There, at them thou tail may toss, A running stream they dare na cross.”