Always always always...all day and all night, which is probably why I can't sleep right now (at 3ish am). I also debate things back and forth with myself or think up scenarios or why this, why that, etc. It drives me up the wall sometimes. I think so much sometimes that I forget to do other things, like homework or chores.
To make things more annoying, I always think visually so if I think about something scary or gross, I can't get it out of my head and start to obsess about it. And even if I've been thinking of the perfect rationale for the longest time about something, I will never properly articulate it; totally suck at describing my thoughts. 