They let us know how flawed our perception is. Study any 2 of these illusions in depth. Be able to answer... What is happening? What is the cause/explanation of this? Enjoy!
http://www.michaelbach.de/ot/
More illusions http://www.ritsumei.ac.jp/~akitaoka/index-e.html
http://www.colorcube.com/illusions/illusion.htm