“The camera does not lie,” the saying goes. And we tend to think of our eyes and our other sensory organs as video equipment, faithfully recording all the details of our busy lives. As you will learn from the articles on illusions collected in this special issue, however, we see with our brains, not with our eyes. And our brains make instant value judgments about the jumble of incoming sensory information, depending on what is important at that moment to us, to create a sensible narrative of the world around us.

Rather than pondering every bit of light that enters our orbs, the brain quickly jumps to conclusions, based on millions of years of evolution. Humans are intensely visual creatures, and we have developed an incredible apparatus for detecting things that are critical to our survival, such as predators, prey and mates. For instance, we can instantly mentally assemble several tiny patches of orange with stripes peeking through dense foliage: “tiger!” As we glance around a room, the image bounces on the retina (the light-receiving tissue at the back of the eye) as various areas of the scene excite different groups of cells. Yet the world appears stable to us, the view a smooth pan across our surroundings. The brain even fills in missing bits of picture in the eye’s blind spot, where the optic nerve pierces the retina.

On the other hand, we do not see everything. Something that is irrelevant to a particular task will not make it to our conscious awareness. In one telling experiment, volunteers had to count how many times a basketball got passed between players. A person in a gorilla suit then strutted across the room. Concentrating on those ball passes, about half the volunteers did not see the gorilla.

Of course, the brain cannot actually tell us about what it is thinking as it processes sensory inputs, focusing on certain items and ignoring others. But our responses to illusions can be just as revealing. Scientists have long used these disarmingly simple—and fun—sensory tricks to probe the mind’s inner workings. This special edition offers an amazing collection of such illusions and the lessons that they teach us about the brain. We can promise you one thing: you won’t believe your eyes.