Built slim enough and small enough to fit in a pocket, The Sudoku Planet contains puzzles arranged by five levels of difficulty. This virtually weightless adjunct to any trip requires only a pencil and an agile mind — then it's time for the fun to begin. Players must add numbers to a 9 x 9 grid, being careful not to repeat digits in any single row or column. Out of this remarkably simple premise comes a fiendishly difficult game, one that keeps minds occupied well after the destination has been reached.