Some illusionists even try to get people to “fall” for these tricks through knowledge of psychology and clever design. One of the better-known, intentional examples is the Rabbit-Duck illusion, which is a sketch of either the head of a rabbit or a duck. The same person can respond that they see different woodland creatures at different times of the year, though this may be because there is no actual reward for saying that you can recognize both, just the expectation that you see something. 

In the case of the rabbit-duck, duck-rabbit, or however you prefer to call it, the brain is creating a full image out of disparate pieces. Like the images here, it’s just our mind trying to figure out what is going on, while filtering everything through what we already know. Unfortunately, our brain can only work with what our senses give it, and once it’s made up its (or our) mind, it might be disinclined to help you see what’s actually happening. So in the case of the two cats, it might not notice the second cat because it’s already ‘established’ that the first cat’s shadow is visible.

Camouflage works in similar ways, by hijacking the methods our mind uses to determine what is what. Once we have accepted that a clump of bushes is a clump of bushes, it will fade into the back of our minds. We might only start to see the hypothetical camouflaged man if we are actively looking for someone or something that may be hiding in the clump of bushes. Animals use this strategy to great effect, while humans, as usual, just copy their homework to replicate the patterns, colors, and shapes found in nature.

Sometimes nature itself conspires to trick and confuse us. For example, a mirage is caused by light bending in such a way that a human, normally depicted as tired and thirsty, sees reflections of things further away or in the sky. Often in popular media, the person, no doubt wandering through the desert, sees an oasis and drags themselves towards it only to find more desert. This is actually not a mirage, simply some sort of psychosis. A real mirage is just the bending of light that can actually be caught on camera. Mirages aren’t limited to the desert and can be found near large bodies of water as well. 

Mother nature wasn’t done with just mirages, certain illusions can also be found near obstructed horizons, where our brains create a slope on a hill that is actually stable. This is sometimes referred to as a gravity hill, though it’s a lot less sci-fi than it sounds. It is also sometimes called a magnetic hill, though neither gravity nor magnets have anything to do with it. It’s caused by surrounding items appearing straight when they are actually leaning, making the ground appear like it’s sloping in one direction or another. 

We sometimes create these illusions ourselves, like the watercolor effect. It comes about when a colored line on a white background appears to ‘bleed’ parts of itself into the white area. Our eyes see this similar to a watercolor painting, where the edges of the brushstroke are lighter than the center. While this actually happens when using watercolors, in the watercolor illusion, the colored lines do not actually have a pale border of any sort. While it is less understood than the others mentioned here, scientists believe it could be a result of our brain ‘splitting’ the image into a background and something placed upon it, i.e. the line. 

The simplest way a layperson can go about making something confusing would be to apply something called contour rivalry, which is also a great band name. Simply put, it’s the process of using shared indents, lines, and groves to create a series of images that change when viewed from different angles. This method has been known for over two thousand years, but creative artists can still use it to trick and confuse viewers.