By the 1990s, sudokus were popular enough to inspire harder variations. Some creators throw letters in the mix, combine as many as 280 grids into one massive field, or layer five puzzles on top of one another to form an overlapping mind-bender. Killer Sudoku adds elements from kakuro, a game that works like a crossword but uses sums instead of verbal clues. Instructions: Fill all rows, columns, and blocks with the digits 1–9 exactly as in a regular sudoku. The catch: The answers inside each region bounded by a dotted line (known as a cage) must total the number there.