This website outlines a different kind of captcha (which is a simple test that humans can pass but computers cannot – typically seen on websites that ask you to verify you are a human by typing in a series of letters from a graphic):
http://www.captcha.net/cgi-bin/pix
You can read more about captcha’s here. I have to admit that there were a few that I couldn’t figure out and I failed. Of the rest, some were kind of challenging.