Bernard Williams' Essay 'The Self & The Future' begins by imagining a straight forward experiment in which two people exchanging bodies. From there Williams then takes his reader through a series of ingenuous steps to show that this is not actually as straight-forward as it seems. By the end of the essay, you don't even know whose who, literally.
If you like philosophical mind-teasers, you can't get any better than this. It took me a while to crack it, and even still I'm not quite sure that my own explanation (which you can read HERE) is adequate.
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