The late Fred Allen produced a 1936 radio broadcast that included a humorous wrapup of the year’s least important events. The exact date the joke formula came into popularity is unknown, but was probably 1936 as an Associated Press newspaper article noted that “Knock Knock” had replaced “What’s This?” as a favorite parlor game. The recipient then repeated the response followed by “Who?” The punster came in with the crusher, a punch line with a pun-based misusage of the set-up word “Isabelle necessary on a bicycle?” The punster initiated “knock knock.” The recipient responded “Who’s there?” The punster provided a variable response (sometimes involving a name, for example Isabelle).
It was a role-play exercise dictated by the punster having a recipient of wit. Knock-knock jokes began as a variation of the format in a children’s game played around 1929. Isabelle who? Isabelle necessary on a bicycle.” In “Cheaper By the Dozen” (a good read in its own right), the dad plays a “Knock Knock” joke with his kids: “Knock knock. I suppose it was in 1949 because the book I read that contained it was published in 1948. My first remembered encounter with that particular form of humor (and punnery) came during a South Dakota summer when I was staying at an uncle’s farm.