My favourite:
Q: "What happens when an irresistible force meets an immovable object?"
A: EVERY force is irresistible, and no object is immovable (for example, even your walking around affects the rotation of the earth in some infinitesimal way). [...corrected]
Other common questions are ones that need you to compute approximate answers on the fly - like the "how many barbers are there in the country" question above, or "how much would you charge to clean all the windows in London?". There is clearly no specific answer to these questions - the employer just wants to see that you can identify all the most important factors (e.g. #men in Britain, #haircuts per year, #haircuts per Barber per day, #working days for a Barber), ballpark the quantities involved (e.g. say 25 million men, 10 haircuts per year, 30 customers per barber shop per day, 5.5 days per week), and combine them in a logical way (... therefore the number of barbers in the UK is around 30,000).
The "random crazy" questions like the "what fruit would you be and why?" one are there only to see how you react to unexpected questions that you don't understand, which, lets face it, come up fairly regularly in most customer-facing jobs. The key is not to let the question phase you. The answer you give is largely irrelevant as long as you are not offensive. If you feel you can answer the question then do so (e.g. "I'm a banana because I'm tough on the outside but soft and sweet on the inside (*vomit*)), or if not then simply say "I'm sorry I have no idea". Whatever you do, greet the question with a smile, and don't let it phase you.
Specific technical questions aside, interviews are all about attitude and reaction. The answers you give are far less important than the way you handle yourself.
