explain vs involve

explain

verb
  • To make something plain or intelligible. 

  • To make plain, manifest, or intelligible; to clear of obscurity; to illustrate the meaning of. 

  • To give a valid excuse for past behavior. 

involve

verb
  • To complicate or make intricate. 

  • To take in; to gather in; to mingle, blend or merge. 

  • To cause or engage (someone or something) to participate or to become connected or implicated. 

  • To comprise or include; to have as a related part. 

  • To envelop, enfold, entangle. 

How often have the words explain and involve occurred in a corpus of books? (source: Google Ngram Viewer )