Knock-knock, who’s there?

Knock-knock is of course the opening gambit and who’s there is the obligatory question, which is then usually answered by a pun based on someone’s name. It was thought to have originated in Britain in the 1930s, but may have derived from an even earlier parlour game popular in Victorian times. It was revived in the 1960s, culminating in the Crack-a-Joke Book published by Puffin in 1978, which devoted a whole chapter to so-called knock-knock jokes.