Imagine you are asked to watch a short video in which six people-three in white shirts and three in black shirts-pass basketballs around. While you watch, you must keep a silent count of the number of passes made by the people in white shirts. At some point, a gorilla strolls into the middle of the action, faces the camera and thumps its chest, and then leaves, spending nine seconds on screen. Would you see the gorilla?

Almost everyone has the intuition that the answer is "yes, of course I would." How could something so obvious go completely unnoticed? But during this experiment at Harvard University several years ago, it was found that half of the people who watched the video and counted the passes missed the gorilla. It was as though the gorilla was invisible.

This experiment reveals two things: that we are missing a lot of what goes on around us, and that we have no idea that we are missing so much.