I love optical illusions. They remind me of the limitations of our sensory engagement with the world and just how much our brains really contribute to processing those inputs. Our brains do ‘fill in the blanks a lot’, exemplified by this paragraph that was prolific on the web in the early 2000s:
Aoccdrnig to a rscheearch at Cmabrigde Uinervtisy, it deosn’t mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.
Net result: reality is in the mind. Animals remind us of those sensory constraints. The commoner garden blackbird has far greater colour-vision/acuity than humans. From its perspective, other blackbirds are a multitude of colours. Perhaps the octopus illustrates the sentiment better though as Peter Godfrey-Smith discusses in Other Minds: The Octopus, The Sea, and the Deep Origins of Consciousness – the octopus is an intelligent creature whose evolutionary path diverged from our own 750 million years ago and its intelligence (aka understanding) vastly differs to our own. Even within our own species, some traits such as dyslexia or colour-blindness have different effects on visual-processing – and subsequently how the world around us is perceived and understood.
Some great optical illusions:
http://www.psy.ritsumei.ac.jp/~akitaoka/o1saishe.html
https://michaelbach.de/ot/
Some word illusions: http://old.marcofolio.net/other/15_cool_word_illusions.html