Tony Whittaker wrote how a 30-second film shows six people playing basketball, three in white shirts and three in black. Volunteers are asked to count how many times the white shirt team passes the ball. At the end of the film, they are asked if they saw anything unusual. Most do not. The unusual thing is: halfway through the film, a man wearing a gorilla suit walks through the players, beats his chest to the camera, and then walks off. 

“When shown the film again, people are utterly surprised to see him, to the extent that they often believe a different film has been substituted for the original one. Their focus on one task has blinded them to a truth.” 

He goes on to say, “This film trick illustrates a simple fact–that if we are only looking for one thing, we do not usually see anything different…The point at which people finally see the unexpected is what [could be called] a ‘gorilla moment.’…So often in life people see only what they want to see and don’t see what they don’t want to see.”