When they came to the part that was now the wood, and turned from the trail to find a camping place, they happened on the spring. "It was real nice," said Jesse with a sigh. "It looked just the way it does now. A clearing, lots of sunshine, that big tree with all those knobby roots. We stopped and everyone took a drink, even the horse."
"No," said Mae, "the cat didn't drink. That's important." (7.3-4)
This is where we learn the most tragic news of all: the Tucks never made the choice to be immortal. They just happened upon their condition. Why? Because they were thirsty. They probably would have preferred a say in the matter, don't you think?