A mother had her child taken from the cradle by elves.
In it's place they laid a changeling with a thick head and staring eyes who would do nothing but eat and drink. In distress she went to a neighbor and asked for advice. The neighbor told her to carry the changeling into the kitchen, set it on the hearth, make a fire, and boil water in two eggshells. That should make the changeling laugh, and if he laughs it will be all over with him. The woman did everything just as her neighbor said. When she placed the eggshells filled with water over the fire, the blockhead said:
Now I am as old as the Wester Wood, but have never seen anyone cooking in shells!
And he began laughing about it. When he laughed, a band of little elves suddenly appeared. They brought the rightful child, set it on the hearth, and took the changeling away.
Jacob and Wilhelm Grimm
Children's and Household Tales (1812)