With an impish smile, the professor announced that he was about to carry out a little experiment. He asked his class to watch a short video of two basketball teams and to count how many times the players in white T-shirts passed the ball. The students found that it wasn't easy to keep their eyes on the moving ball, but most of them believed they counted correctly.
After the show, the teacher turned to face everyone again: "What did you think about the gorilla?" There was a shocked silence. He restarted the video, and after a few seconds a collective groan rippled through the room: as the audience now realized, a person in an ape costume had walked right across the court, pausing in the middle to pound on his chest.