How did tucks realize the spring had caused their condition?

The Tuck family realized that the spring had caused their condition when they discovered they had not aged after drinking from it. Initially, they experienced the joy of eternal life, but over time, they recognized the implications of their immortality, including the inability to change or grow. This realization was further solidified when they observed the world around them continuing to change while they remained the same. Their understanding deepened as they grAppled with the consequences of their choice to drink from the spring.