If the price of seeing each other again is goodbye, then stay together forever. Song Rang told Chen Shi a new definition of goodbye and made him a promise, but with the worn-out paper, the promise gradually became invalid. The worn-out paper was locked in the small wooden box year after year. When they met again, the promise had an echo.
Intention: If the promise gradually expires, there may be an echo as the person who made the promise comes back.
