Do you ever zone out while reading?

I'm sitting here reading and all of a sudden, I zone out and start thinking about something completely unrelated to my book. By the time I come back to, I had already read half the page and have no clue what I just read, so I have to reread it agian. Then as I reread it, I start vaguely remembering what I had read.

How can I continue reading and not be conscious about it while thinking about something completely different? Has anyone else done this? Explain! It happens all the time and kind of weirds me out.