Hey Diddle Diddle is a nursery rhyme.

Rhyme

Hey diddle diddle,

The cat and the fiddle,

The cow jumped over the moon,

The little dog laughed to see such fun,

And the dish ran away with the spoon.

Tolkien's version

J. R. R. Tolkien invented (by back formation) the imagined original ditty that is recorded in the simplified nursery rhyme. The title of this version as given in The Adventures of Tom Bombadil, is "The Man in the Moon Stayed Up Too Late".