My favorite* online math puzzle contest — Monday Math Madness — has decided to use a problem that I submitted for a previous contest.
CHECK IT OUT HERE!
*It’s really the only one that I know. But that shouldn’t be taken disparagingly; the problems are just hard enough that you have to make one really rewarding conceptual leap, and just easy enough that with enough perseverance you know you can conquer it.
Continuous Everywhere but Differentiable Nowhere 
Congrats on getting picked. :-)
There is also Math Minute which is sometimes fun but sometimes too easy.
Thank you for a fun puzzle!
@Kate: I’ll check it out!
@Denise: Thanks your your thanks! [Takes a bow.] I wish I could take credit, but it goes to Mathcamp!
Thanks! Depending on the point of view, you gave me two wonderful hours – or stole me two hours;). Maybe I’ll even post my solution…