Obvious?
White 1 seems to make a 2nd eye immediately. What's so hard about this problem?! But then Black's tesuji at 2 makes White's 2nd eye very difficult! Through 7, White's moves are forced. After Black 8, White has a choice. See the next diagram for White's continuation.

See the SGF for these and many more variations.

Continuation diagram.
White 9 is a marvelous tesuji! By leaving the 2 Black stones on the bottom edge, White can catch Black in a shortage of liberties. Black cannot wriggle away from White capturing the 2 bottom stones with atari with 15. White 17 makes the 2nd eye!

Black variation.
Black can try to avoid the previous diagram with 10. But with 13, White makes one last tesuji to take advantage of Black's shortage of liberties. White 15 makes the 2nd eye in the corner.

White variation.
Capturing immediately with White 9 doesn't work. Black now has time for the throw-in at 10 to eliminate White's last chance for a 2nd eye. After Black 12, White's chance for a 2nd eye are gone.

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