Toreador (6/24/2010)
A good idea, but a bit too easy to guess! Even though I failed to come up with an example, I got it right as the obvious answer was 'not possible', therefore for there to be any point asking the question, the answer had to be 'possible'!
I wondered whether that reasoning would be common and perhaps lead to a success rate close to 100%. I thought that if the success rate looked too high I could devise a question where the obvious answer was the correct one for my next attempt and see if it attracted a high failure rate. But the success rate turne out to be reasonable - 246 people (about a third of those trying it so far) have got it wrong, so perhaps not too easy after all.
Tom