When you think about something which leads you
to think about
something else, then that thought will lead to another thought, and so on until you are
realizing what is happening.
I had a
thought chain recently, it went like this:
I was thinking about sex, then I started to think
about me being a father, then I started to think about my future, then I started to think about my future career, then I thought about me being a hobo, then I started to think about me dying until I
wondered why I was thinking about dying. Then it all came to me...