A pun is a humorous use of a word that has two meanings, or of words with the same sound but different meanings. For example, the joke 'When is a door not a door?' 'When it's ajar' depends on the fact that 'ajar' sounds like 'a jar'.

Read on and check out these funny examples:

1. Let’s talk about rights and lefts. You’re right, so I left.
2. Time flies like an arrow. Fruit flies like a banana.
3. When a clock is hungry it goes back four seconds.
4. A boiled egg every morning is hard to beat.
5. Two fish are in a tank. One says to the other, “Err...so how do you drive this thing?”
6. I went to buy some camouflage trousers yesterday but couldn't find any.
7. I've been to the dentist many times so I know the drill.
8. Being struck by lightning is a shocking experience!
9. Without geometry, life is pointless.
10. A chicken crossing the road is truly poultry in motion.
11. The roundest knight at King Arthur’s table was Sir Cumference. He acquired his size from far too much pi.
12. I went to a seafood disco last week....and pulled a mussel.
13. She had a photographic memory but never developed it.
14. Two antennas met on a roof, fell in love and got married. The ceremony wasn't much, but the reception was brilliant!

Source: http://www.funenglishgames.com/funstuff/funnypuns.html