Optical illusions are the best way to prove that what we think we see, is often not the truth. Our brain tends to make assmptions about the world, by altering, omitting or adding up the information it receives via the senses.
Even though these sensations might feel accurate, they might not reflect the sourounding reality. After all, all external stimuli are processed by the same parts of the brain which are also responsible for dreams and halucinations.
Moreover, a phenomenon described as "the Bayesian brain", shows how the information that already exists in the brain (the so called "Priors") will "overwrite" your perceptions, trimming or building on the recieved information, so that it will match the “priors” already encoded in the brain.