Tsumego (Capture) #5309415 (1-move)
Black to play — capture the marked white stones
Capture these stones
Why this move works
The answer is A2. The target white stones (A1, B1) have only one liberty (dame) left — A2. They are in atari.
Playing A2 fills that last liberty, takes the group to zero liberties, and captures both stones. Counting a group’s liberties first is the foundation of capturing problems.