Huh? What's Sudoku?

Sudoku puzzles are played on a 9x9 grid. At the start of the puzzle, some of the cells will already be filled in with numbers. The challenge is to fill in the empty cells, placing a single digit from 1 to 9 in each cell. Your goal is to completely fill in the grid so that, when finished, every row, every column, and every 3x3 subgrid contains each of the digits 1–9 once and ONLY once.

Each Sudoku has a single, unique solution that can be reached through careful logic, no guessing required. The attraction, and what has made it an international phemonenon, is that the rules are so simple that virtually anyone can play, yet the solutions can be so complex as to require extensive logic and reasoning. Each Sudoku has a rated level of difficulty so that puzzlers can opt for more challenging puzzles as they become more experienced.