Pun

A pun is a joke which is a "play on words." People make puns in order to make other people laugh. Many jokes are actually puns. 
A pun usually uses a word which can have more than one meaning, even if the spelling is different: 

Sometimes a pun may use a whole phrase that can be heard in more than one way, as in the following knock-knock joke:

 “Knock Knock!”
 “Who’s There?”
 “Dishwasher.”
 “Dishwasher who?”
 “Dishwasher way I ushed to shpeak before I got my falsh teesh”.
(The last sentence is supposed to mean: “This was the way I used to speak before I got my false teeth”).

There is a joke about a man who sent ten different puns to friends in the hope that at least one of the puns would make them laugh. Unfortunately, no pun in ten did.

(The joke here is on “no pun intended,” which is said when a person unintentionally says a pun by coincidence, and announces the find ).

There are many websites and books which have jokes, very often with puns.

Puns are easier to make in some languages than others. Languages with many homophones (words which sound the same but have a different meaning) such as the Chinese language are most suitable for puns.

Puns do not have to be about homonyms. They can sometimes be a double entendre using both a literal sense of the word and a metaphor (figurative use). For example, if a young man joins the Navy and someone says that he "sailed through his exams", this is a joke because: 1) the expression "to sail through something" means "to do it easily", but 2) the Navy is about sailing in ships. It is possible to say something like that without realizing that it is a pun. 

Another example is a story about two boys growing up and eating more and more food. One of the parents just began a new job and the other says, "I hope you can start bringing home the bacon!" In this example, bacon is a play on words meaning both money and the food, bacon.  

Many British people love making puns, especially bad puns (where a word has to be mispronounced to make the joke work). Lewis Carroll made many puns in his books Alice in Wonderland and Alice through the Looking Glass.

These are also a few examples of different puns:

Really cool songs were heard as the rock band played on the iceberg,

"You're fired!" said the boss to the man shot from a gun.
Category:Figures of speech
Category:Humor

"How is your vineyard doing?" a neighbour asked. The woman replied, "It's growing grape!"  (play on words: "It's going great!")

Helium, Neon and Argon float into a bar.
The bartender says:
"Sorry, but we don't serve your kind."
They don't react.
(Helium, Neon and Argon are noble gases and they don't perform chemical reactions easily. This is why argon is used in lightbulbs.)