Tsumego (Capture) #5309420 (1-move)
Black to play — capture the marked white stones
Capture these stones
Why this move works
The answer is F6. The three white stones (E7, F7, G7) sit along the top edge with their only liberty at F6.
Playing F6 captures all three. Stones crawling along the edge have liberties on one side only, so a single key point brings the whole group down.