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The web filter loophole that became a school institution. Cool Math Games' "math" branding kept it off the blocked-site list while every other gaming site got nuked, making it the de facto arcade of every school computer lab in the mid-2000s.
Remember when sudoku grids were suddenly everywhere — newspapers, airport bookstores, that one waiting room where time moved differently? The mid-2000s delivered the last great pencil-and-paper puzzle obsession, and it trained a generation of commuters to genuinely fear the word "fiendish."