We are great at reading characters that have been distorted to the point they baffle computers. When you sign up for a new email account they make you interpret those characters to be sure you are not a computer program signing up for multiple accounts to be used for sending spam. The technique is called "CAPTCHA" and was invented by Luis von Ahn, see this great NOVA Video about his ingenious work.
Ahn, then a grad student at Carnegie-Mellon and now a professor there, solved another tough problem using a variation of CAPTCHA. Google and others are scanning old books and digitizing the text. The problem is words in old books with obsolete typefaces may be misaligned or smudged or otherwise distorted to the point they can't be reliably read by computers. That is where what Ahn calls "reCAPTCHA" comes in!
The word "legume" in the above figure is from some old book and can't be reliably read by the computer. So, as part of the CAPTCHA process of signing up for an email account, reCAPTCHA presents two words. One is the distorted CAPTCHA word that the computer knows and the other is a word copied from some old book. If the human can correctly interpret the distorted word, they figure he or she can interpret the word from the old book correctly as well. Since millions of people sign up for various computer accounts every day, Ahn has put us all to work helping computers digitize the text of old books!
PS: Thanks to my son-in-law David for putting my wife Vi and me on to this NOVA video!
PPS: In the video, a talking computer is asked: "What is a perfect date?" The answer is "June 23, 1912". I know why. Do you?