For a more technical definition:
From Wikipedia, the free encyclopedia
A paraprosdokian // is a figure of speech in which the latter part of a sentence, phrase, or larger discourse is surprising or unexpected in a way that causes the reader or listener to reframe or reinterpret the first part. It is frequently used for humorous or dramatic effect, sometimes producing an anticlimax. For this reason, it is extremely popular among comedians and satirists. Some paraprosdokians not only change the meaning of an early phrase, but they also play on the double meaning of a particular word, creating a form of syllepsis.
And now for some samples:
1. Where there's a will, I want to be in it.
2. The last thing I want to do is hurt you. But it's still on my list.
3. Since light travels faster than sound, some people appear bright until you hear them speak.
4. If I agreed with you, we'd both be wrong.
5. We never really grow up, we only learn how to act in public.
6. War does not determine who is right - only who is left.
7. Knowledge is knowing a tomato is a fruit.. Wisdom is not putting it in a fruit salad.
8. To steal ideas from one person is plagiarism. To steal from many is research.
9. I didn't say it was your fault, I said I was blaming you.
10. In filling out an application, where it says, 'In case of emergency, Notify:' I put 'DOCTOR'.
11. Women will never be equal to men until they can walk down the street with a bald head and a beer gut, and still think they are sexy.
12. You do not need a parachute to skydive. You only need a parachute to skydive twice.
13. I used to be indecisive. Now I'm not so sure..
14. To be sure of hitting the target, shoot first and call whatever you hit the target.
15. Going to church doesn't make you a Christian any more than standing in a garage makes you a car.
16. You're never too old to learn something stupid.
17. I'm supposed to respect my elders, but its getting harder and harder for me to find one now.