Riddles with a mathematical twist I am looking for riddles that are understandable for everyone(so especially non-mathematicians) but require mathematical knowledge or deep abstract ideas to be solved.
The best answer will be the riddle that is most understandable( especially not contain any abstract math at all, so for example fermat's last theorem is NOT what I am looking for) but most mathematically demanding at the same time.
(I hope it is clear how I will try to objectively assess the answers, so that no one has to vote for closing this thread)
Also it would be nice, if the riddle you recommend is not very famous.

Although not needing deep mathematical knowledge...


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*A blindfolded man is handed a deck of 52 cards and told that exactly 10 of these 
cards are facing up. How can he divide the cards into two piles (possibly of different 
sizes) with each pile having the same number of cards facing up?


An old-fashioned implication one:


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*Mrs Claus always sneezes just before it starts snowing. She just sneezed. 
“This means that it’s going to start snowing”, thinks Santa. Is he correct? 


Robbed these from http://math.alamzy.com/wp-content/uploads/2012/10/Handbook.pdf
