daydreaming vs castle in Spain

daydreaming and castle in Spain both are nouns.

Word	Noun	Adjective	Verb	Adverb
daydreaming	Yes	No	No	No
castle in Spain	Yes	No	No	No

As nouns, daydreaming and castle in Spain are synonyms defined as:

Other synonyms of daydreaming include air castle, castle in the air, daydream, oneirism, reverie, revery.

daydreaming (noun)	castle in Spain (noun)
absentminded dreaming while awake	absentminded dreaming while awake