Flags change a number
Take away the flags around a number: what is left is how many mines it still needs.
Once a mine is flagged, it counts for every number around it. A 3 with one flag still needs two mines, so it works like a 2 on its other cells. If it has exactly two hidden cells left, both are mines.
How it works
The highlighted number already has flags around it (yellow).
Your turn
Each board is a real position from a game. Tap or click to open; right-click, long-press or switch on Flag to place a flag. If you are not sure, take a hint: the first one only shows where to look.
Which hidden cells can you be sure of?
How the exercises were chosen
A computer checked every board: in what you see, the only cells one number can prove belong to a number with flags around it, which has as many hidden cells as mines once the flags are taken away.