to do this is the fact that in chapter 25 the toad is being pestered by a fairly aggressive dog. Winnie pours the spring water over the toad so it never has to worry about being attacked by that dog, or any dog, ever again.  

Winnie pulled out the cork from the mouth of the bottle, and kneeling, she poured the precious water, very slowly and carefully, over the toad. . . Then she stooped and put her hand through the fence and set the toad free. "There!" she said. "You're safe. Forever."

Jesse gave Winnie the bottle of magic spring water with the intent that she would drink it when she turned 17.  That way they both could be together at 17 forever.  How romantic.  

The first thing that Winnie does with the bottle is hide it.  She does not want her nosy parents finding it, so Winnie puts it down at the bottom of one of her bureau drawers.  

The next time that the bottle makes a big appearance in the story is when Winnie pours the contents of the bottle over the toad.  The toad is being pestered by a dog, and Winnie rushes over to the fence to rescue it from the dog.  At this point in the story, Winnie isn't 100% sure whether the spring water does indeed give eternal life or not, but she pours the water over the frog anyway, because the thought that the frog is now safe forever is a nice thought to Winnie.  

Then she stooped and put her hand through the fence and set the toad free. "There!" she said. "You're safe. Forever."

Winnie pours the spring water onto the toad.

Winnie accidentally sees Jesse drinking from the spring in the woods near her house and ends up going on a grand adventure. She learns that he is immortal, and so is his entire family.  Jesse is seventeen, but he is really one hundred and four years old.  He will be seventeen forever.

Jesse is quite smitten with Winnie, even though she is not old enough yet to be a girlfriend.  He asks her to wait until she is his age and then drink the water and become immortal too.

But the thing is, you knowing about the water already, and living right next to it so's you could go there any time, well, listen, how'd it be if you was to wait till you're seventeen, same age as me—heck, that's only six years off—and then you could go and drink some, and then you could go away with me! (Ch. 14)

Winnie is “struck dumb” by this offer.  Winnie also likes Jessie quite a bit.  She is too young to make a decision about who she wants to spend the rest of her life with, let alone eternity.  Jesse tells her to think about it.

While helping Mae Tuck escape from prison, Jesse gives Winnie a bottle of the spring water and reminds her of her options.  Later, Winnie sees a toad she has befriended being harassed by a dog.  She decides to get the bottle Jesse gave her and use it to help the toad.

The toad still squatted where she had dropped it, the dog still waited at the fence. Winnie pulled out the cork from the mouth of the bottle, and kneeling, she poured the precious water, very slowly and carefully, over the toad. (Ch. 25) 

The decision is not necessarily as meaningful as you might think.  At the time, Winnie felt that she could get more water any time.  A fire in the woods eventually eliminates that possibility, but Winnie has made her choice.  She wants to be mortal.  She grows up and dies, and leads a normal life.  Her experience with the Tucks seems to show her that being immortal is not all it’s cracked up to be.  Dumping out the water that Jesse gave her was a hint at her future choice not to be immortal. 

Before the Tuck family departs, Jesse gives Winnie "a bottle of water from the spring" and instructs her to drink it when she turns seventeen.  With this water, Winnie will be able to live forever like Jesse and his family.  Winnie had wanted to live forever.  She did not want to die.  Her opinion changes, however.  When she gets back home, she hides the bottle of spring water "in a bureau drawer."  

Later, Winnie sees a dog bothering a toad.  She chases it away and then goes up to her room to fetch the bottle of spring water.  She impulsively "[pulls] out the cork from the mouth of the bottle" and dumps it onto the toad, causing it to live forever.  Winnie reasons that there is still eternal water in the spring in the woods.  

Winnie never drinks the water from the spring.  She lives to be almost eighty-years-old before she dies.  Jesse and his mother return to look for her in 1950, but they discover that she had already passed away.  They later see the toad, who had been the recipient of the bottle of spring water, sitting in the road.