One winter's day, when the ground lay deep in snow, a poor boy was sent to the forest with a sled to bring back wood. After gathering the wood and loading it onto to the sled, he was so cold that instead of going straight home, he thought he'd make a fire and warm himself a bit. He cleared a space, and as he was scraping away the snow, he found a little golden key.
"Where there's a key," he said to himself, "there's sure to be a lock."
So he dug down into the ground and found an iron box.
"There'must be precious things in it," he thought. "If only the key fits!"
At first he couldn't find a keyhole, but then at last he found one, though it was so small he could hardly see it. He tried the key and it fitted perfectly. He began to turn it--and now we'll have to wait until he turns it all the way and opens the lid. Then we'll know what marvels there were in the box.
(Tale 200 in The Complete Grimm's Tales for Young and Old, translated by Ralph Manheim).