Optical illusion photos challenge our sense of reality; they make us realize that what we see is simply our brains' best interpretation of the visual cues our eyes provide. Our brains have adapted not to see what is actually there, but what we deem as useful to see.

These eye tricks occur when a visual scene unfolds that is different from what our brain has decided is the norm, and it will respond in odd and often confusing ways until finally arriving at a conclusion.

According to Exploring optical illusions, there are 3 main types of optical illusion:

Literal illusions create images that are different from the objects that make them, like most of the entries on this list. For example, if 3 birds are flying, 2 above one, it can form the illusion of a smiling face.

Physiological illusions, such as afterimages following bright lights or adapting stimuli of excessively longer alternating patterns (contingent perceptual aftereffect), are presumed to be the effects on the eyes or brain of excessive stimulation of a specific type - brightness, tilt, color, movement, etc. The theory is that stimuli have individual dedicated neural paths in the early stages of visual processing, and that repetitive stimulation of only one or a few channels causes a physiological imbalance that alters perception. 

Cognitive illusions are assumed to arise by interaction with assumptions about the world, leading to "unconscious inferences", an idea first suggested in the 19th century by Hermann Helmholtz. Cognitive illusions are commonly divided into ambiguous illusions, distorting illusions, paradox illusions, or fiction illusions.

1. Ambiguous illusions are pictures or objects that elicit a perceptual 'switch' between the alternative interpretations. The Necker cube is a well known example; another instance is the Rubin vase.

2. Distorting illusions are characterized by distortions of size, length, or curvature. A striking example is the Café wall illusion. Another example is the famous Müller-Lyer illusion. 

3. Paradox illusions are generated by objects that are paradoxical or impossible, such as the Penrose triangle or impossible staircases seen, for example, in M. C. Escher's Ascending and Descending and Waterfall. The triangle is an illusion dependent on a cognitive misunderstanding that adjacent edges must join. 

4. Fictional illusions are defined as the perception of objects that are genuinely not there to all but a single observer, such as those induced by schizophrenia or a hallucinogen. These are more properly called hallucinations.