head-scratcher vs jigsaw

head-scratcher

noun
  • Something particularly puzzling or confusing. 

  • A device used to scratch the head. 

jigsaw

noun
  • Something that can be compared to a jigsaw puzzle. 

  • A power saw with fine teeth and a narrow blade which can cut curves in wood or metal. 

  • A jigsaw puzzle. 

verb
  • To cut something using a jigsaw. 

How often have the words head-scratcher and jigsaw occurred in a corpus of books? (source: Google Ngram Viewer )