Surround to capture
Capture is the core hook of the game: occupy every liberty of a stone or group and it comes off the board at once. A centre stone needs four enemy stones to capture, an edge stone three, a corner stone only two — geometry makes corners both the most efficient and the most fragile ground, the same fact behind the proverb gold corners, silver sides, grass centre. Captures feed the score: under area scoring (the default here) your territory plus your surviving stones count; under territory scoring the prisoners subtract from the opponent directly. Either way the lesson is the same — a stone with no room to breathe is a stone already lost. Black to play: take the corner stone.