My Thoughts Run Blank

Have you ever read something but didn't absorb any of the information you just read, so you just read the words without comprehending them? It's happened to all of us, so there's no pleading innocent in this case. There may be different instances where this happens but I think it happens to me mostly when I'm reading something, but my mind is thinking about something different. Then in the midst of reading I realize that I'm not understanding what I am reading, so the knowledge that is provided isn't being explained to its fullest potential. Its mostly because of how the reader reads it and how he/she comprehends what they just read.