What did Mae place into her pocket in tuck everlasting?

In "Tuck Everlasting," Mae Tuck places a small, golden, and ornate device called a "tuck" into her pocket. This device is a kind of special, ancient key that symbolizes the Tuck family's secret of immortality. It represents their connection to the enchanted spring that grants eternal life. Mae's action signifies her desire to protect this secret and the life they lead as a result of it.