First, you need to watch the following video. See how well you can do with the experiment. You'll have to watch very intently, as it's a bit tougher than it looks.
Ok, how did you do?
For an explanation to the science behind the video, and to explore some other possibilities (inattentional blindness, amnesia, agnosia among others) to what is going on your brain here, swing by to read the full science article on Actionnooz.