knock on wood vs think about

knock on wood

verb

  • To take a customary action to ward off some misfortune that is believed to be attracted by a presumptuous statement. 

intj

  • Hopefully; a self-directive to undertake the customary action to ward off bad luck. 

think about

verb

  • To consider as a course of action. 

  • To ponder. 

How often have the words knock on wood and think about occurred in a corpus of books? (source: Google Ngram Viewer )