Natalie Babbitt reveals the mystery of the spring very slowly in Tuck Everlasting. When Winnie Foster first tries to drink from it in Chapter 5, she has run away from home, at least for the day, and sees Jesse Tuck drink from the hidden spring. Jesse will not allow her to have any water, and to prevent her from drinking it, Ma and Miles kidnap Winnie when they arrive.
The reason Winnie cannot drink from the spring is hinted at in Chapter 1. There readers learn that if anyone noticed the spring that bubbled up under the tree despite the pebbles that concealed it, "that would have been a disaster so immense that this weary old earth, owned or not to its fiery core, would have trembled on its axis like a beetle on a pin." In other words, Winnie cannot drink from the spring because it will produce a worldwide cataclysm. But readers do not know what that horror is until Chapter 7, when the Tucks explain to Winnie that their immortality began when they drank the waters of the spring.
(The entire section contains 2 answers and 620 words.)