In chapter 5 of Tuck Everlasting, Winnie is happily wandering through the woods, when all of a sudden she sees a handsome young man lifting up a pile of pebbles to reveal a bubbling spring of water.
The young man, who is Jesse Tuck, takes a drink. Unbeknownst to Winnie, the spring of water is an elixir of youth; anyone who drinks from it will become immortal, just like Jesse and his family.
The Tucks' spring is highly sought-after by the man in the yellow suit, an unscrupulous character who sees the elixir of youth as nothing more than a money-making opportunity. Not unreasonably, the Tucks want to protect their water from him and from anyone else who might want to misuse it for their own gain.
They hide their special water source by piling up pebbles on top of it. Not only will this ensure that unsavory individuals like the man in the yellow suit won't get their greedy little hands on it, but also that unsuspecting people like Winnie won't accidentally stumble upon it and make themselves immortal without having the chance to think through the consequences.
In fact, that's precisely what Winnie very nearly does. It's only the sudden arrival on the scene of Jesse's mother and brother that prevents her from becoming immortal like them.