He nods when she asks whether he understands death. That isn't the difficult part of this concept. He definitely does not understand the last bit, and replies with an appropriately confused look, followed by a brief moment of thoughtful silence. It's a disquieting idea, for more than one reason.

He almost doesn't want to ask. But he needs to ask. He hopes she'll understand the question. "Was anyone here dead already before they got here?"