Thoughts are ephemeral things that follow the same quantum mechanics rule as Schrödinger's cat. That is, attempting to note the train of thought in progress, even just mentally, interferes with it and therefore changes it. Not noting the train of thought in progress means that the whole train is often lost back into the subconscious, with only fragments left over that aren't sensical enough to record.
For example, on the way to work today, I had part of a train of thought that went thusly:
1. Han Chinese as opposed to non-Han Chinese. (I have no idea how I came to that subject.)
2. There are non-Han Japanese. The original inhabitants of those islands have physical characteristics distinct from the present inhabitants. Some of them still remain on Hokkaidō.
3. The original inhabitants of the American continents most closely resemble Asians. The dominant modern view seems to be that all of the American Indians are physically and culturally homogeneous - but really, if one thinks about it, that makes no sense. The Americas are very large, and there is no reason why all the peoples on them would be the same. (And in the same way, neither are all of the peoples in Asia, even though a lot of people think all Asians look the same, too.)
4. Out of all of the tribes that were here before, my favorite were always the Lakota. I was obsessed with Indians as a preteen, reading everything I could find about them (which wasn't much, unfortunately). I once drew a pencil portrait of Sitting Bull, based off of the famous one of him (and possibly the only picture of him that exists). I wonder what happened to it...
5. Something about blood quantas and how they make no sense. A person is not bits and pieces of different ethnicities and cultures. A person is a single whole. There is a Lakota ritual, apparently, that declares an outsider to be Lakota - and once it's done that's the end of the question; regardless of what the person looks like or what their blood heritage is, they are fully Lakota thereafter.
I spent much of my time at work today trying to recall why I was thinking about any of this and how I could turn it into a post worth reading. Sadly, I had no luck, and that's why it's just a list of fragments.