[Most old jokes, particularly ancient jokes, aren't that funny. Certainly the world's oldest recorded joke doesn't raise many chuckles these days:
Something that's never been known since time immemorial: a young lady who doesn't break wind in her husband's lap.
This had them giddy with mirth in ancient Sumer (1900-1600 BC), but while the theme of flatulence is durable, the gag itself stinks. The same can be said of the oldest documented joke in England, an Anglo-Saxon riddle cited in the 10th century manuscript, The Exeter Codex:
Question: What hangs at a man's thigh and wants to poke the hole that it's often poked before?
Answer: A key...
A fool broke wind while in bed with a deaf person. When the latter caught the smell and began to complain, the fool said: "Come on, how could you hear it if you're supposed to be deaf?"
If you thought a theme was beginning to emerge, you'd be right. Some topics lend themselves to joking, and social taboos are particularly fertile ground. Issues that civilised society deems unacceptable can be safely addressed within the context of humour, and that's one of the reasons we enjoy, and need jokes.
There's something about an old joke, or a classic comic scene, that – once we're aware of its status – can work like a laugh track, coaxing laughter out of us. This could also explain why some jokes have instantly recognisable forms and structures. Consider this, from the Philogelos:
A student dunce goes to the doctor and says, "Doctor, when I wake up, I'm all dizzy, then after half an hour I'm OK."
"Well," advises the doctor, "wait a half-hour before waking up."
This is the first recorded "doctor, doctor" joke, and it's another ancient gag that's still funny.]