What’s the quietest thing you’ve ever noticed in a child?

I was watching a kid—just four, maybe—sit perfectly still during naptime, eyes closed but not asleep. Her thumb was pressed against her lower lip, like she was tasting the air. No breath, no shift. I stood there for three minutes just trying to decide if she was breathing at all. That silence wasn’t absence. It was something else. What’s the quietest thing you’ve ever seen a child do that made you stop?