Why can't Winnie drink from the fountain in Tuck Everlasting?

I believe that the question is asking about events that happen in chapter five of Tuck Everlasting.   Winnie is attempting to give herself some space from her parents, so she escapes into the nearby forest. She quickly comes upon the spring in question, and she sees Jesse Tuck taking a drink from it as well.  Winnie is immediately infatuated with Jesse Tuck, but she is also cautious of why he is in her family's woods.   Winnie eventually gets around to asking Jesse about the spring water.  She assumes it must be good, and Winnie makes a move to drink from the spring too.  Jesse does everything in his power to convince Winnie that the spring water is no good to drink at all.  

"Believe me, Winnie Foster," said Jesse, "it would be terrible for you if you drank any of this water. Just terrible. I can't let you."

Winnie is unconvinced, but any further attempts made by her to get a drink are interrupted by the arrival of Miles and Mae Tuck.  Winnie is then quickly spirited away by the Tucks, and she never has another chance within the pages of the text to drink from the actual spring.