Understanding Zero-knowledge proofs through illustrated examples

Each cell must contain a number (1–9) that is unique to the row, column, and 3×3 gridTo do so, Alice builds a tamper-proof machine that executes the proof to Bob and friends. Of course, the machine prevents Bob from simply flipping over the cards in their cells. The cards are shuffled and no longer have the original orderA few verification rounds later, Bob and friends are convinced that Alice has solved this puzzle.

Source: blog.goodaudience.com