What is the source of the tucks immortality?

The source of the Tuck family's immortality in "Tuck Everlasting" is a magic spring that they accidentally discovered. When they drank from the spring, it granted them eternal life, preventing them from aging or experiencing death. This immortality comes with significant consequences, as they are unable to partake in the natural cycle of life and death, leading to feelings of isolation and a longing for the normal human experience. The story explores the implications of living forever and the value of mortality.