Andrew Lang 
The White Duck and Other Fairy Tales 

Once upon a time a great and powerful King married a lovely Princess. No couple were ever so happy; but before their honeymoon was over they were forced to part, for the King had to go on a warlike expedition to a far country, and leave his young wife alone at home. Bitter were the tears she shed, while her husband sought in vain to soothe her with words of comfort and counsel, warning her, above all things, never to leave the castle…