Inattentional Blindness: The Invisible Gorilla

Two teams pass basketballs around; your job is to count the passes made by the team in white. Halfway through, a person in a full gorilla suit walks into the middle of the frame, faces the camera, thumps her chest, and walks off — about nine seconds on screen. Roughly half of the people counting passes never see the gorilla. Told about it and shown the tape again, many insist it must be a different video. Daniel Simons and Christopher Chabris ran the study in 1999 under the title 'Gorillas in Our Midst'; the clip has since been watched tens of millions of times, and it still works on people who know something is coming.