Minesweeper

Minesweeper is played on a grid of covered squares. Some of them hide mines; the rest are safe. The goal is to uncover every safe square without opening a mine.

Beginner is a 9 by 9 grid with 10 mines.

• Open a square to see what is under it. A mine ends the game.
• A safe square shows a number: how many of its eight neighbours (sideways and diagonally) hold a mine. A square with no mine around it opens its neighbours for you, so whole areas clear at once.
• Put a flag on a square you know is a mine. Flags are only notes for you; they don't count toward winning.
• The game is won when every square without a mine is open. The counter above the board shows the mines minus the flags you have placed.

Every number is a small promise about the squares around it. A 1 with only one covered neighbour means that neighbour is a mine. A 1 that already touches a flagged mine means all its other covered neighbours are safe.

Most of a game is these two checks, done again and again: does this number already have all its mines, or does it have exactly as many covered neighbours as mines left? Either answer gives you sure squares to flag or open.

Start along the edge of the open area, where numbers touch only a few covered squares. Small numbers next to few covered squares are the easiest to read.

Flag a mine as soon as you are sure of it. A flag doesn't win anything, but it makes the next numbers easier to read, and it lets you chord: tap the number to open the rest of its neighbours in one go.

Don't rush the last few squares. With only a handful of mines left, the counter can settle a square that no number reaches.

Click or tap a square to open it. Right-click, or press and hold on a phone, to put a flag on it or take it off. Flag mode below the board makes every tap a flag, which is easier on a small screen.

Tap a number whose mines are all flagged to open every other square around it at once (this is called chording). If one of your flags is wrong, that opens a mine, so flag with care.

Your first click is always safe. Each board starts at a cell marked with a gold ring: wherever you click first, that area opens, and from there the whole board can be solved by reasoning. Hint shows the next sure square and why; press it again to open or flag it. On a keyboard, the arrow keys move, Enter or Space opens, and F places a flag. Your game is saved in this browser.

Games about finding hidden mines on a grid appeared on home computers in the early 1980s. The version most people know, with numbered squares, flags and three standard sizes, spread in the early 1990s when it came installed on a popular desktop operating system.

In 2000 a mathematician proved that deciding whether a Minesweeper position is even possible is NP-complete, one of the famous hard problems of computer science. That is part of why random boards often end in a forced guess, and why we check every board here first.