"Hey, Diddle Diddle" (also "Hi Diddle Diddle", "The Cat and the Fiddle", or "The Cow Jumped Over the Moon") is an English nursery rhyme. A version of the rhyme is: Hey diddle diddle, the cat and the fiddle, the cow jumped over the moon; the little dog laughed to see such sport, and the dish ran away with the spoon. The rhyme is the source of the English expression "over the moon", meaning "delighted, thrilled, extremely happy".

Source: Wikipedia