Two eyes to live

If a group can be surrounded, why does anything survive? The answer is the most important shape in Go: a group with two separate holes inside it, called eyes.

The other player can only fill one point per turn, and placing a stone inside an eye with no liberties of its own is not allowed — unless it captures. With one eye, the attacker can eventually fill the outside and then the eye itself, capturing everything. With two separate eyes, the attacker would need to fill both at once, which no single move can do. The group can never be taken.

Strong players spend much of a game asking one quiet question about each group on the board: can it make two eyes? A big group that cannot is weaker than a small one that can.