Website for Mathematics enigmas? I seek some website just for the pleasure of solving mathematics enigmas.
I know this website : Brilliant.org I just want to know if you know some others good sites ! 
Thank you

Making some advertisement for my own website:
Mathcounterexamples
It is not about mathematics enigmas, but about mathematics counterexamples. Which are often puzzling. You can look at post titles and try to find examples having the required properties.
For example find an unbounded continuous function over $[0,+\infty)$ having a finite integral. One example: here.
