What the numbers mean
Every number counts the mines in the eight cells around it. That is the whole rule.
An open cell with a number touches up to eight cells: left, right, above, below and the four corners. The number says how many of them hold a mine. A cell at the edge of the board has fewer neighbours, and its number counts only those. Open cells are never mines, so a number is always about the hidden cells around it.
How it works
Look at the highlighted 1. Of the eight cells around it, all but one are open.
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 are next to a 1 with a single hidden cell, and these are exactly the mines you flag.