Tsumego (Capture) #5309424 (2-move)
Black to play — capture the marked white stones
Capture these stones
Why this move works
The answer is C1 then A2. The three white stones (A1, B1, B2) have two liberties: A2 and C1.
The key is to play C1 first. If you fill A2 first, White extends out to C1 and escapes. Block the escape side (C1), then fill A2 to capture.