It's an old riddle, with a subtle answer. Richard Feynman gave an account of his days at Princeton, where freshman were often accosted by sophomores with such questions. Part of Fe... moreIt's an old riddle, with a subtle answer. Richard Feynman gave an account of his days at Princeton, where freshman were often accosted by sophomores with such questions. Part of Feynman's genius was figuring things out in advance, so that he seemed to have the answer to impossible questions almost instantly.
Hint to the answer - when you look at your image in the mirror, what is hidden from view?
Update August 15, 2016: Mathologer (a YT channel that I just discovered and to which you should all subscribe, if you love math) has a detailed video on this less
