Since I got my new glasses, DVD cases on shelves look suspiciously small. At first I wondered if it was some new smaller design of DVD case. However, DVDs themselves are obviously still the same size, and inspecting a case reveals that it’s only just wide enough to encompass the diameter of the disc. OK, you may think, no mystery here. New glasses. However, my new glasses are exactly the same prescription as the old ones.
Some people believe that they perceive the world as it actually is. There are many experiments that can disprove this notion. For instance, take a look at Edward H Adelson’s checker shadow illusion. To me, the two squares A and B look so obviously different that if I didn’t know it was an optical illusion, I would never pause to think that they might be exactly the same color. Similar experiments can demonstrate that your hearing is just as subjective.