"The Origin of Elves"
Once, God Almighty came to Adam and Eve. They welcomed him gladly and showed Him everything they had in their house, and they also showed Him their children, who seemed to Him to be very promising. He asked Eve whether she had any other children besides the ones she was just showing him. She said, "No." But the truth of the matter was that Eve had not yet got around to washing some of her children and was ashamed to let God see them, and so she had pushed them away somewhere out of sight. God knew this, and said: "That which had to be hidden from Me, shall also be hidden from men."
So now these children become invisible to men, and lived in woods and moorlands, knolls and rocks. From them the elves are descended, but human beings are descended from those of Eve's children whom she did show to God. Human beings can never see elves unless the latter wish it, but elves can see men and enable men to see them. It is for this reason that the elves are called the Hidden People.
--an Icelandic story, from Scandinavian Folktales, ed. Jacqueline Simpson.