When the brain gets a set of visual data, it immediately starts processing it a certain way, according to some automatic processes that are more hardwired than we tend to realize.
For instance, in the picture below, there is a clear image of a man's face. But you may have a hard time finding it. Keep looking.
Do you give up? Because it's a very clear picture of a man's face.
Don't keep reading unless you want a clue.
Okay, here's your clue.
It's near the bottom edge of the rectangle, about one third over from the left side.
See it? Now that you see it, you will never have any trouble seeing it again. Your visual cortex was just processing "beans" until you saw the face, even though you knew there was a face to look for. Your "face"-recognizing neurons weren't actually working. Now that you've seen the face, your face-neurons are kicked in gear with this particular stimulus and there's nothing difficult at all about seeing the face.
Check out some more really cool face illusions from Scientific American.