I can get to the “correct” solution by playing a2 followed by b1, but then what’s to stop white from playing a1 and taking the stone I just played?
Black has no move after that, and it seems like they can’t take the corner? I assume I am missing something basic.
Edit: Here is a link to Puzzle 5: Play Go at online-go.com! | OGS
Link to puzzle?
Ahhh, yup, sorry. It’s this one: Play Go at online-go.com! | OGS
It’s a ko. Black can kill if he wins the ko.
But… I mean, you’re saying black has to have a threat on the board that would force white to play elsewhere. But then they’re comparing that threat to losing the corner, right?
It just doesn’t seem like a good puzzle to me if the result is the possibility of winning the corner, not actually winning it. 
Well you can win the game because this possibility exists.
Even if you were winning the game by 1 or 2 points, and you were forced to give up a couple of stones not to lose the corner, you lose the game.
If you have to choose between giving some points up somewhere else on the board, or losing the whole corner, that’s the point of this puzzle and the point of puzzles that involve ko.
^^ is definitely the point 
There’s an unwritten hierarchy to tsumego solutions: If you can live outright, that’s the answer, if you can’t but you can get seki, that’s the answer, and if you can’t but you can get ko, that’s the answer. Then within each category there can be better or worse solutions by points, and some less common categories, but those 3 are the basic idea
Thanks, I think this was what I was missing. I guess I thought these were all about life.
I appreciate the responses, everyone.
