Sudoku-also known as number place-is a logic-based placement puzzle. The aim of the puzzle is to enter a numerical digit from 1 through 9 in each cell of a 9 X 9 grid made up of 3 X 3 sub-grids (called "regions"), starting with various digits given in some cells (the "givens"); each row, column, and region must contain only one instance of each numeral. Completing the puzzle requires patience and logical ability. Although the puzzle was first published in a US puzzle magazine in 1979, it initially caught on in Japan in 1986 and attained international popularity in 2005. Sudoku is known to be NP-complete, so if you feel very confident about doing 9x9 problems, then you can move up to 25x25 or 36x36 and start suffering all over again. All the supposedly "different" solution strategies seem to reduce to different ways of handling the data for backtracking-called "what if in Sudoku lore.