build castles in the air vs make believe

build castles in the air

verb
  • To have any desire, idea, or plan that is unlikely to be realized; to imagine visionary projects or schemes; to have an idle fancy or a pipe dream; to daydream. 

make believe

verb
  • To pretend or imagine. 

noun
  • The act of pretending that what is imaginary is real. 

adj
  • imaginary; conjured in someone's imagination, especially when imagined by a child 

How often have the words build castles in the air and make believe occurred in a corpus of books? (source: Google Ngram Viewer )