If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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Another one that seems of similar difficulty to Cheryl's birthday is http://en.wikipedia.org/wiki/Ages_of_Three_Children_puzzle (the article contains spoilers):
What are the ages of the children?
I actually remember finding blue eyes easier than three children, but I don't remember my relative ages when I first heard them, and I discussed both with my family at the time. But blue eyes had an "obvious" approach which worked after routine application, and three children/Cheryl require you to extract a different insight from every step of the puzzle.