The Kakuro book said “Expert.” You believed it for the first ten minutes, while the corners filled in nicely. Now you’re looking at a block in the middle of the grid where every clue has five ways to be right, your pencil marks are three deep, and the eraser has worn a soft gray …

You can finish a hard Sudoku in ten minutes. You spot X-wings without trying, and naked pairs jump out at you. So you flip to the Kakuro page expecting a warm-up, and twenty minutes later you’ve placed four digits, erased three of them, and you’re refusing to write a 5 in a cell because …