Our Creator did not provide our brain with an index. As a
result, we usually are not aware of what we know on any
particular topic. “Do you know anything about electric trains?”
“No. Well, wait — I had a Lionel set when I was 11. The tracks
had three rails. I remember seeing some other kinds in my
neighborhood hobby store. Oh, yeah, and …” 20 minutes later, you
realize you do know something about electric trains. And given
more time, you’d discover more.  Neuroscientists are making
great strides in understanding how we remember, but the process
is still mysterious in many ways. Without understanding how it
works, clustering gives us access to our knowledge so that we
can make a list of what we know and don’t know about any topic.
This is useful in many situations — coming up with designs;
problemsolving; preparing for a meeting or a presentation;
making a shopping list; and many more.  Gabriele Rico devotes an
entire book to clustering: Writing the Natural Way. Highly
recommended.  So, how do you cluster? He