The silence after someone says 'I'm fine'

It’s not the words. It’s the pause that follows — like they’ve just handed you a key to a room you weren’t meant to enter. I’ve heard it in the library, in the checkout line, once from a kid who returned a book with a highlighter still inside. The way their voice dropped at the end, like they were testing the weight of the lie. I don’t know why it sticks. Maybe because I’ve spent years learning to read between the lines of overdue notices and dog-eared pages, but never learned how to hear what’s not said.