Feeling tired all day? Let’s talk about some jokes and have some fun.
First, here is the oldest joke in the world, from Sumeria, 1900BC. "Something which has never occurred since time immemorial; a young woman did not fart in her husband's lap."
Um. What?

What about this from Egypt, 1600BC? "How do you entertain a bored pharaoh? You sail a boatload of young women dressed only in fishing nets down the Nile and urge the pharaoh to go catch a fish."
Still can’t get it?
Try this from Britain, 1000AD. "What hangs at a man's thigh and wants to poke the hole that it often poked before? Answer: A key."
I bet these jokes must make some of you laugh. But why can’t these jokes amuse everyone?

The missing social context may ruin a joke
Scientists suggest that from ancient times to 21th century, many popular jokes have a similar structure. Jokes may involve a sense of superiority, violations of social expectations, or mismatch between a concept and a situation. For example, the third joke above is a sex gag. In real situation the answer is key but you may think of something else in your mind.
Although many popular jokes have a similar structure, there are always some people not laughing. This may be because you don’t know the social context.
As for the first Sumerian joke, I must admit it I have no way of knowing why the Sumerians actually found the joke funny, because I am a modern American instead of a four THOUSAND YEAR old Sumerian.

But when I change that farting joke into “We hold these truths to be self-evident: who smelt it denied it.” Americans over there, does this make you smile? I actually giggled when I typed that.
The thing is, all jokes contain culturally shared values to some extent. They meant to amuse people who share the same values with them. So, people from different social groups will be amused by different jokes.
Time and ages may ruin a joke

You must know Charlie Chaplin. He is perhaps the most famous comedian in the world. To this day, westerns still appreciate Chaplin’s artwork, but to be frank, I don't often find his comedy funny. It just seems too predictable and old-fashioned.
Besides, when it comes to humor, older people are often not at the same level with younger people.
“What do you call a sleeping dinosaur? A dino‐SNORE.” My 15-year-old daughter laughs so loudly when she heard this, while I just thought it’s a flat joke.

If you don’t believe me, you can also share the accounts your kids follow on YouTube and see if those “superstars” can make you laugh.
Brain networks may affect a joke
Since humor is associated with social bonding and cohesion, certain brain networks may affect your appreciation of jokes. Some scientists suggest that the superior frontal gyrus in your brain is an essential part. Stimulating this region may make a flat joke funnier.

But other scientists argue that this region also affects your ability of doing other social and linguistic tasks, which means your sense of humor may be overlapping with your social and linguistic ability.
To conclude, you should first figure out the social and cultural complexities of a joke, then you may be able to know why some people feel it funny while others feel dull.