to be so interested in your own thoughts, your brain stops focusing on the
reception from your senses. You only
snap out of it when, 1) you realize you
zoned out and subsequently "come back into the real world", 2) something happens that catches your attention, like say a major movement in front of your face, or 3) someone says your name.
When the principle came into to talk to us about Jacob running away from school on his
longboard, I was so bored by that
old hag that I had to zone out in five seconds. I
snapped out of it when she stopped talking, and guess what I was thinking about? A sandwich.