In the old days, anybody interested in seeing a Mets game during a trip to New York would have to call the team, or write away, or wait to get to the city and visit the box office. No more. Now, all it takes is to find an online ticket distributor. Sign in, click “Mets,” pick the date and pay.

But before taking the money, the Web site might first present the reader with two sets of wavy, distorted letters and ask for a transcription. These things are called Captchas, and only humans can read them. Captchas ensure that robots do not hack secure Web sites....

One of the wavy words quite likely came from a digitized image from an old, musty text, and while the original page has already been scanned into an online database, the scanning programs made a lot of mistakes. Mets fans and other Web site users are correcting them. Buy a ticket to the ballgame, help preserve history.

The set of software tools that accomplishes this feat is called reCaptcha and was developed by a team of researchers led by Luis von Ahn, a computer scientist at Carnegie Mellon University....