Here's a typical example of my brain in action: Once, when I was a little kid, I decided to pray about something bad I'd done, which was probably either stealing my sibling's toy or taking our lord's name (Sailor Moon) in vain. But one sentence in, I got distracted thinking about something funny someone at school said, which made me think about school and how cute one of my boys in my class was, and how if we got married, his dumbass friends wouldn't be invited to the wedding. It was like a late-night Wikipedia trip where you start by looking up the history of Saturday Night Live in the '80s, and an hour later you end up trying to find out what the largest prehistoric mammal was.
I kept trying to go back to the prayer, and by the time I was finished, I was actually mentally exhausted. And weirdly enough, the reverse has happened as well. Once I was reading an article, and didn't stop until I heard yelling. I looked up and realized that the room was filling with smoke -- my sister had gone for a walk and asked me to watch the chicken she was cooking. Not only did I forget to check on it, but I had been so focused on the article that I didn't notice anything else, even when the room was about to burn down around me.
This is because of something called "hyperfocus." People who suffer from ADHD are known specifically for their lack of focus, but there's also a problem with getting incredibly locked in at other times, seemingly at random. There's no middle ground; when my brain feels like finally focusing on something, someone could shout in my face and I'd take in none of it. When my brain doesn't feel like focusing on something, I can spend hours reading a page over and over, gathering no information other than "Yes, this is a page."