What do you do when someone says 'I'm fine' and you know they're not?

It’s the silence after that. Not the kind that’s comfortable, but the one that sits heavy in the room like a stone. I’ve learned to listen past the words—how their voice cracks on the ‘fine’, how they avoid eye contact, how they keep turning the cup in their hands. I don’t fix it. I just stay. Sometimes I say nothing at all. But I’m still there. What do you do? Just… stay?