Inside the puzzles

How does a computer make a puzzle with exactly one solution, and how does it know how hard the puzzle is? Watch it happen on a real puzzle, step by step.

How a Sudoku is made

Every Sudoku starts empty. First the computer fills the whole grid: that becomes the solution.

Step 1 of 234

Speed

Space plays and pauses, ← and → go one step, Page Up and Page Down jump between chapters.