Easy-to-See, Hard-to-Solve Problems

This has been around for awhile, but still contains some interesting problems to play with... Responses to the Quora question, "What are some unsolved problems in math which seem easy at first glance?":

http://www.quora.com/What-are-some-unsolved-problems-in-math-which-seem-easy-at-first-glance

Meanwhile, ICYMI, I've posted a bit of a year-end blog retrospective over at MathTango:
http://mathtango.blogspot.com/2015/01/proverbial-look-back.html

...and hope before month's end to have 1-2 new interviews over there as well.