Illusions are surprising, intriguing, and fun. They also give us a peek into the amazing principles that determine how our brain works.
The most common illusions are optical. Illusions are formed because perception is a dynamic process controlled by the brain; it is not a simple "photograph" of reality. The brain and sense organs analyse information coming from the environment, and they focus on whatever best preserves and utilises the brain's pre-existing knowledge about the world. The brain gets the information from the senses, processes it, and combines different types of information from different senses. It can ignore some details and emphasise others, and it can even change the sensitivity of the sense organs. What all this means, simply put, is that the relationship between perceived reality and actual reality is quite complex.