What is a “knock knock joke”?
It’s a joke with a specific format that requires interaction between two people. The person saying the joke begins with the phrase “knock knock” and the other person responds with “who’s there?” The person asking the joke then gives the setup, and the other person repeats back the setup verbatim and adds the word “who” at the end. The person asking the joke then delivers the punchline. An example of a knock knock joke is: 
Knock knock
Who’s there?
Orange
Orange who?
Orange you glad I’m only doing one knock knock joke!?