Unless you've been living under a rock, you've probably heard — and told — many knock-knock jokes. Knock-knock jokes are very common in all English-speaking countries. They're popular in many places around the world, including the United Kingdom, Ireland, France, Belgium, Australia, Canada, South Africa, the Philippines and India.

A type of pun or play on words, the knock-knock joke is a "call and answer" exercise in which there's a person who tells the joke (the protagonist) and a person who plays along (the antagonist). The basic five-step format of the knock-knock joke is well-known:

1. The protagonist: “Knock, knock!" 2. The antagonist: “Who's there?" 3. The protagonist: 4. The antagonist: plus “who?" 5. The protagonist: plus a pun or play on words

Here's an example of a popular knock-knock joke:

Knock-knock! Who's there? Dwayne. Dwayne who? Dwayne the bathtub, I'm dwowning! (a play on “drain the bathtub, I'm drowning!")

It seems like knock-knock jokes have been around since the beginning of time, and no one really knows how they came about. There are a couple of theories, but neither has proved to provide definite proof of the joke's origins.

Some people believe knock-knock jokes developed as a result of “call and answer" routines that castle guards used to identify people after dark in the Middle Ages. When people would approach the castle after dark, they would have to call out to the guards to get inside.

The guards would answer, “Who's there?" They would then carry on a back-and-forth conversation about their identity and reason for wanting into the castle. Guards must have occasionally teased people with this routine, perhaps to alleviate the boredom of keeping watch throughout the night.

Others believe knock-knock jokes may have gotten their start with William Shakespeare's Macbeth. In this famous play, a funny character delivers a long, humorous speech that follows the familiar pattern of the modern knock-knock joke.

Today, knock-knock jokes can provide more than just a chuckle. Developmental psychologists now study young children telling knock-knock jokes to assess their language development skills!

Here are a couple more examples of popular knock-knock jokes:

Knock knock! Who's there? Little old lady? Little old lady who? Hey! I didn't know you could yodel!

Knock, knock! Who's there? José José who? José can you see by the dawn's early light? (a play on “Oh say can you see by the dawn's early light?")

Wonder What's Next?

You can count some things on your fingers and toes, but there aren’t enough fingers and toes on Earth to count what we’ll be counting in Wonderopolis tomorrow!