Most people are probably familiar with the story of the Elves and the Shoemaker; how a humble cobbler discovered night after night that the shoes he intended to make the next day were made for him, and after he and his wife discovered that small naked men were doing the work for them, they made a set of clothes for each to thank them. The elves were pleased with this, took the clothes, and disappeared. Interestingly, although the original publication from 1812 did emphasize how quickly and nimbly the elves worked, there was nothing mentioned about how excellent the workmanship was until later versions (Originally, the first pair of shoes made more money simply because they fit well). Also, whereas the first version simply ended with the elves dancing right out the door and never returned, the Grimms later assured readers that the shoemaker still prospered the rest of his days.
(Later illustrators often gave the elves a ragged set of clothes to avoid depicting naked men)
What many people might not realize is that that story is included in a set of three short tales in the Grimms' collection, all about people's different interactions with elves. The second story is about a servant girl who was asked to be the godmother to one of the elves' children. Her employers advised her that she should do as the elves requested, so she saw their beautiful kingdom where everything was made with precious stones and materials. She meant to go home after performing her duties as godmother, but the elves requested that she stay with them for three more days. After that, they filled her pockets with gold and she returned to the house to work-only to find out that time passes differently in the land of the elves; the three days with the elves had actually been a whole year, in the first edition. The Grimms must have wanted to emphasize the creepy factor by later making it seven years, and adding that her employers had died during that time.
The final story is about a changeling, that had been placed in a child's crib by the elves. Taking the advice of her neighbors, the mother boiled water in eggshells, causing the changeling to laugh and therefore lose his power. The elves then returned the rightful child and took the changeling away with them.
It's an interesting group of tales that show the different sides of elves in folklore-sometimes kind and helpful, other times viciously stealing your baby-or sometimes your worlds cross and your world is forever changed, even though there appeared to be no evil intent in asking the servant girl to stay an extra three days. (I also wonder what would have happened if the maid had refused to act as godmother...)