Watching someone disappear
It really is like hearing the flutter of a bird's wing to watch the fleeting smile cross the face of someone whose mind has moved outside your reach. It reminds me of my grandfather when I was twelve and he was beginning to forget who he was. It's funny: except for a few times, he always remembered me.
We also found out that he could, too, sing, though for years he insisted he was tone deaf. A fine strong baritone, and I heard many songs that soldiers sang in the AEF. Not all of them appropriate for a 12-year-old girl, but there you go.