system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 why a person with A.D.D (ADHD) is unable to focus on something like studying, but can have full focus on something non productive?
gpt
The disorder isn't always that you can't focus on anything at all. It's that the part of your brain that lets you *control* what you focus on is broken. So sometimes, you really need to focus on something and your brain decides it just won't. Other times, the thing it decides to fixate on is the least important thing and you can't make it focus on anything else. 

If a person with ADHD could control that, they wouldn't have ADHD.