Der goldene Schlüssel
The concluding story from the fairy tale collection Children's and Household Tales, compiled by the Brothers Grimm. The tale ends openly; a nice metaphor invites the reader to discover the contents of the casket with their own key.
In the wintertime, when a thick blanket of snow lay on the ground, a poor young boy was forced to go out on a toboggan and fetch some wood. Now, when he had found and gathered some together, he wished, because he was freezing so, not to go straight home but first to make a fire and warm himself a little.
Translated by me from the German "Projekt Gutenberg" e-text. More can be found here.
So he scratched away at the snow, and while he was clearing the ground thus, he found a small golden key. Now he thought: where the key was, its lock must also be, and he dug into the ground and found an iron casket.
"If the key would but fit!" he thought. "There are certainly expensive things in that casket." He looked, but there was no padlock there. At last he found one, but it was so small that it could scarcely be seen. He tried, and fortunately the key fit. Then he turned it round, and now we must wait until he has quite unlocked it and lifted the lid, and then we shall find out what wonderful things lie in that casket.