THE BEST SUDOKU: Every puzzle in this calendar is created by Nikoli, the best Sudoku makers in the world--not a computer--to balance toughness and solvability and keep you hooked. TRAIN YOUR BRAIN: The week begins with the easiest puzzles and the difficulty increases day by day. Just in case you need them, solutions are printed on the back of the pages. ​NOW PLASTIC-FREE!: Page-A-Day(R) Calendars are environmentally friendly! Printed on responsibly sourced paper and 100% recyclable, with sturdy cardboard backers.