"I think I understand what you mean," she said in a mature voice.

"Really?"

"Um," she answered. "There are some things in this world that can be done over, and some that can't. And time passing is one thing that can't be redone. Come this far, and you can't go back. Don't you think so?"

I nodded.

"After a certain length of time has passed, things harden up. Like cement hardening in a bucket. And we can't go back anymore. What you want to say is that the cement that makes you up has hardened, so the you you are now can't be anyone else."