An appropriate knock-knock joke for small children uses "Lettuce" as the reply to "Who's there?" The stinger at the end is "Lettuce in. It's cold out here."
A pair of knock-knock jokes appeals to little children's enjoyment of repetition. In the one, the first reply is "Amos," and the final line is "A mosquito." In the follow-up joke, the initial answer is "An udder." The final response is "An udder [another] mosquito."
Knock-knock jokes originated during the early 1900s, and by the middle of the 1930s they were very popular with both children and adults. Another joke for children introduces "Canoe" in the first part and finishes with "Canoe come over?" As popular as the jokes were, they also had many detractors who felt the puns were overused and a foolish fixation.
Children enjoy the quick turnarounds that occur in knock-knock jokes. For instance, when the joke-teller first replies "Who" and the listener asks, "Who who?", the child happily responds with another question: "Are you an owl?" Another animal knock-knock joke has the punch line "Goat to the door and find out."
In the 21st century, knock-knock jokes are still prevalent, and they continue to have admirers and haters. Books dedicated only to knock-knock jokes are sold. Some specialize in specific topics, such as holidays or Minecraft.