Imagine if your Nalgene were filled with water that would make you live forever. Pretty sweet deal, right? Well, that's Winnie's situation, and it's put her in kind of a pickle.
First, let's recap. Before he prepares to run out of town with his family, Jesse gives Winnie a bottle of immortality juice—i.e., water from the spring. It's pretty simple: if Winnie drinks it, she'll live forever, just like the Tucks.
This is one hefty temptation, wouldn't you say? It reminds us a little of the serpent in the Garden of Eden who encouraged Eve to eat the fruit from the tree of knowledge. (For more about that, check out our discussion of "Religious Allegory.") From that moment, the immortality water comes to symbolize the choice that Winnie will have to make.
She makes the first choice pretty impulsively. After running to get the water and pouring it on the toad, "she stooped and put her hand through the fence and set the toad free. 'There!' she said. 'You're safe. Forever'" (25.17). She's setting him free, just like she wanted to be free at the beginning of the book.
Sure, she still has the power to go back to the spring and get some later-in-life immortality. But are we supposed to assume that she won't? After all, if the water bottle represents the temptation and the choice, once she's given it to her toad friend, maybe she's made a more important choice than we think.