I think the root of humor comes from an expectation that is unfulfilled. I have absolutely no research to back this up, so I am merely speaking from a stream of thought. It seems like the mind has certain expectations for every speaker and every human for that matter. When someone or something fails to meet those expectations, that’s what leads to laughter. Humor can take many forms, but from what I can tell, it seems to take an idea that is simple and applicable to the audience and it combines it with a completely unexpected detail. The presentation of an idea and detail catches the audience off guard which seems to trigger a reaction of laughter.
For example, when someone tells a knock-knock joke, the first few lines are relatively expected. “Knock-knock” “Who’s there?” Here, nothing unexpected arises until the initial idea is revealed: “Little old lady.” At this point, there are many questions arising in the mind of the audience. They can only draw conclusions on things they already have thought about before such as a grandmother or a fairy tale character. They are now in a state of curiosity and or have given the idea very little thought. They respond “Little old lady who?” Now, they may not have realized something potentially funny about what they said which makes the punch line genuinely unexpected: “Wow, I didn’t know you could yodel!” This gives them the satisfying answer they have been waiting for the entire joke. If he had said “Little old lady drinks water,” this is much more expected and factual which fails to stimulate the mind. The initial idea of the old lady was completely transformed into a yodeling line with the addition of one word and I think this unexpected transformation is what makes something funny.