The Secret History Of Knock-Knock Jokes : link

http://www.npr.org/blogs/npr-history-dept/2015/03/03/389865887/the-secret-history-of-knock-knock-jokes?lang=en&utm_campaign=10today&flab_cell_id=2&flab_experiment_id=19&uid=21271311&utm_content=article&utm_source=email&part=s1&utm_medium=10today.0303&position=4&china_variant=False


Pmc