How am I able to read multiple pages in a book while simultaneously thinking of something completely different
Unconscious multi-tasking. The brain actually has more processing power than the waking mind can use so with practice the brain can do things automatically while you focus on something else.
People do this all the time without realizing it. Like walking and talking or when you remember something an hour later. Martial artists and some athletes train to do this since reacting without thinking is faster than reacting while thinking. Your brain can also analyze a situation and come to conclusions without your mind realizing it. It’s where “Gut Feelings” come from.
Out of curiosity, are you also able to remember and comprehend what you just read or does it get buried in the unrelated thoughts?
it’s a cool thing your brain does called covert attention! wikipedia has better explanation than I could muster so here you go:
“Covert orienting is the act of mentally shifting one’s focus without moving one’s eyes. Simply, it is changes in attention that are not attributable to overt eye movements. Covert orienting has the potential to affect the output of perceptual processes by governing attention to particular items or locations… does not influence the information that is processed by the senses.”
source: ‘Attention’ wiki page: ‘overt and covert orienting’ subsection
I’d like to know this too because I used to read books ALOUD to my students while thinking of other things all the time! It was like I had two brains. One that was reading out loud and the other that was planning lessons for 15 minutes later.