dr_blurb (676176) writes "If you like your logic and number puzzles to be of the harder variety,
check out "The 10 Hardest Logic/Number Puzzles": we've tried to identify
the World's hardest puzzles in categories like Sudoku, Kakuro, and chess.
There are 9 on the page so far, so your suggestions are needed for the
10th entry. And what is it that makes these puzzles so difficult?"