If anyone following this post would like to try the same exercise (minus the "walking around Oxford" component, naturally), I would be interested in hearing how it proceeded and what one's thought processes were in the comments below. (But please don't just post the solution, which can after all be found in mere seconds using an internet search.)
Anyway, here is a formulation of the puzzle. I do not know the actual provenance of the puzzle, or its original wording; the wording below is taken from an xkcd discussion forum, which can be easily located via search engine if desired.
You are the most eligible bachelor in the kingdom, and as such the King has invited you to his castle so that you may choose one of his three daughters to marry. The eldest princess is honest and always tells the truth. The youngest princess is dishonest and always lies. The middle princess is mischievous and tells the truth sometimes and lies the rest of the time.
As you will be forever married to one of the princesses, you want to marry the eldest (truth-teller) or the youngest (liar) because at least you know where you stand with them.
The problem is that you cannot tell which sister is which just by their appearance, and the King will only grant you ONE yes or no question which you may only address to ONE of the sisters. What yes or no question can you ask which will ensure you do not marry the middle sister?