Thor went home to Thrudvang, but the flint-stone still stuck fast in his head. Then came the vala whose name is Groa, the wife of Orvandel the Bold. She sang her magic songs over Thor until the flint-stone became loose. But when Thor perceived this, and was just expecting that the flint-stone would disappear, he desired to reward Groa for her healing, and make her heart glad. So he related to her how he had waded from the north over the Elivogs rivers, and had borne in a basket on his back Orvandel from Jotunheim; and in evidence of this he told her how that one toe of his had protruded from the basket and had frozen, wherefore Thor had broken it off and had cast it up into the sky, and made of it the star which is called Orvandel’s toe. Finally he added that it would not be long before Orvandel would come home. But Groa became so glad that she forgot her magic songs, and so the flint-stone became no looser than it was, and it sticks fast in Thor’s head yet. For this reason it is forbidden to throw a flint-stone across the floor, for then the stone in Thor’s head is moved.

- Prose Edda (Thor and Hrungner)