SuDoku
SuDoku, also known as Su Doku, purports to be an ancient and venerable Japanese pastime - like a crossword, but using numbers as symbols. It is not mathematics; it requires just (moderately) simple logic, and above all concentration.
For some, it has become a consuming passion, fuelled over the past year by the publication in UK national newspapers of a daily example. I suspect it has resulted in as many man-hours lost to employers as the General Strike . . . to say nothing of broken sleep, crockery and marriages.