Popular Tales from the Norse, by George Webbe Dasent, , at sacred-texts.com
THERE was once a girl who used to go to the river to fetch water, but when she went she was never in a hurry to come back, but stayed so long, that they made up their minds to watch her. So one day they followed her to the river, and found when she got there she said something (the reciter forgets the words), and a fish came up and talked to her; and she did not like to leave it, for it was her sweetheart. So they went next day to the river to see if the fish would come up, for they remembered what the girl said, and used the same words. Then up came the fish immediately, and they caught it, and took it home, and cooked it for dinner,—and a part they set by, and gave it to the girl when she came in. Whilst she was eating, a voice said, "Do you know what you are eating? I am he you have so often talked with. If you look in the pig's tub you will see my heart." Then the voice told her to take the heart, and wrap it up in a handkerchief, and carry it to the river. When she got to the river she would see three stones in the water; she was to stand on the middle stone and dip the handkerchief three times into the water. All this she did, and then she sank suddenly, and was carried down to a beautiful place, where she found her lover changed from a fish into his proper form, and there she lived happily with him for ever. And this is the reason why there are mermaids in the water.