Each Sudoku is made of 81 cells, divided into 9 rows, 9 columns, and 9 boxes. The boxes are 3 cells by 3 cells and are marked off by the thicker lines. Some of the cells contain a digit, 1 through 9. The object is to fill the empty cells so that the digits 1 through 9 appear exactly once in each row, exactly once in each column, and exactly once in each 3 by 3 box.