Further your learning!
Optical illusions are fun! Here are some sites to enjoy and explore:
http://www.optillusions.com/
http://www.moillusions.com/
http://www.123opticalillusions.com/
http://www.echalk.co.uk/amusements/opticalillusions/illusions.htm
http://www.eyetricks.com/illusions.htm
http://kids.niehs.nih.gov/illusion/illusions.htm
http://www.sandlotscience.com/
http://www.skytopia.com/project/illusion/illusion.html
http://www.optillusions.com/
http://www.moillusions.com/
http://www.123opticalillusions.com/
http://www.echalk.co.uk/amusements/opticalillusions/illusions.htm
http://www.eyetricks.com/illusions.htm
http://kids.niehs.nih.gov/illusion/illusions.htm
http://www.sandlotscience.com/
http://www.skytopia.com/project/illusion/illusion.html