My father is 71, and since March he has watched the same cartoon episode 61 times. I know because I’m on his streaming account. Every time I open it, his profile says Continue watching: Season 1, Episode 9. That’s all. He used to watch the news and old westerns. I called my sister. She said her father-in-law did something like this before his diagnosis. So I booked a neurologist and told Dad it was a routine checkup. He sat through the whole memory test without complaint. He repeated the word list backward. Afterward the doctor told me, “Honestly, he’s sharper than I am.” In the car I asked him straight. “Dad. Why that episode?” He looked out the window for a while...