knowing the rules doesn't mean you know how to play.
A great
post in the
Panorama blog with a riddle in MDX. No knowledge in MDX is required for this riddle, because they teach you what you need to know to solve this riddle.
Have fun. Believe me - you will.
Update: Look at
Mosha Pasumansky's blog for another review at this MDX riddle. He claims that the answer in Panorama's Blog is not complete. While I'm new to MDX, I understand that their answer is not 100% complete, but I think it's enought in order to make their point.