This is much more complicated than it seems at first. Give it a try. Go to this site and follow these instructions:
When viewing the video, try to count the total number of times that the people wearing white pass the basketball. Do not count the passes made by the people wearing black.
You can learn more about the demo here.
Rambles Blog at Star Chamber 
So here’s the trick: did you notice that during the video, a person in a black gorilla suit walks through the middle of the scene, faces the camera, thumps his chest, and then walks off the other side? The whole thing is an exercise in what is called “inattentional blindness”. Because you’re concentrating on something else, you don’t notice even something as odd as a person in a gorilla suit. Apparently, about half of the people tested with this video DO spot the gorilla, but it’s still pretty funny.