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.

Puzzles based on Ramsay Theory could qualify. For example, there are 9 people in a room for a meeting. Amongst any three there is at least one pair who have never met before. Show that there is a group of four people amongst the nine who were mutual strangers before the meeting.
