A "knock-knock joke" is a type of humor that typically involves a call-and-response format. One person starts by saying "knock, knock," prompting the other person to ask, "Who's there?" This exchange continues with the first person providing a name or phrase, leading to a punchline that often involves a pun or play on words. These jokes are popular among children and are often used to encourage laughter and interaction. They are simple to understand and can be easily adapted with various themes or characters, making them a fun way to engage with friends and family.