A joke is a question, short story, or depiction of a situation made with the intent of being humorous. To achieve this end, jokes may employ irony, sarcasm, word play and other devices. Jokes may have a punchline that will end the sentence to make it humorous.
A practical joke or prank differs from a spoken one in that the major component of the humour is physical rather than verbal (for example placing salt in the sugar bowl).
Jokes are typically for the entertainment of friends and onlookers. The desired response is generally laughter; when this does not happen the joke is said to have "fallen flat".
However jokes have other purposes and functions, common to comedy/humour/satire in general.
Antiquity of jokes
Jokes have been a part of human culture since at least 1900 BC. According to research conducted by Dr Paul McDonald of the University of Wolverhampton, a fart joke from ancient Sumer is currently believed to be the world's oldest known joke. Britain's oldest joke, meanwhile, is a 1,000-year-old double-entendre that can be found in the Codex Exoniensis. 
A recent discovery of a document called Philogelos (The Laughter Lover) gives us an insight into ancient humour. Written in Greek by Hierocles and Philagrius, it dates to the third or fourth century AD, and contains some 260 jokes. Considering humour from our own culture as recent as the 19th century is at times baffling to us today, the humour is surprisingly familiar. They had different stereotypes, the absent-minded professor, the eunuch, and people with hernias or bad breath were favourites. A lot of the jokes play on the idea of knowing who characters are:
A barber, a bald man and an absent minded professor take a journey together. They have to camp overnight, so decide to take turns watching the luggage. When it's the barber's turn, he gets bored, so amuses himself by shaving the head of the professor. When the professor is woken up for his shift, he feels his head, and says "How stupid is that barber? He's woken up the bald man instead of me."
There is even a version of Monty Python's Dead Parrot sketch: a man buys a slave, who dies shortly afterwards. When he complains to the slave merchant, he is told: "He didn't die when I owned him." Comic Jim Bowen has presented them to a modern audience. "One or two of them are jokes I've seen in people's acts nowadays, slightly updated. They put in a motor car instead of a chariot - some of them are Tommy Cooper-esque."
Psychology of jokes
Why we laugh has been the subject of serious academic study, examples being:
- Immanuel Kant, in Critique of Judgement (1790) states that "Laughter is an effect that arises if a tense expectation is transformed into nothing." Here is Kant's 220-year old joke and his analysis:
"An Englishman at an Indian's table in Surat saw a bottle of ale being opened, and all the beer, turned to froth, rushed out. The Indian, by repeated exclamations, showed his great amazement. - Well, what's so amazing in that? asked the Englishman. - Oh, but I'm not amazed at its coming out, replied the Indian, but how you managed to get it all in. - This makes us laugh, and it gives us a hearty pleasure. This is not because, say, we think we are smarter than this ignorant man, nor are we laughing at anything else here that it is our liking and that we noticed through our understanding. It is rather that we had a tense expectation that suddenly vanished..."
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