Captchas are those distorted snippets of text that some web sites use to try to prevent spammers and bots from automatically registering accounts. The idea is that a human can read the words, but a computer cannot, and so only a human can fill out the text box correctly.
However, it’s probably possible to get humans to crack the captcha for fun. Imagine a “missile defense” typing tutor and game, where each incoming missile is a captcha text. If you type the text, it goes away. Since the game can’t tell what the texts are to begin, it would send the same word to multiple players and only block the “incoming missile” when multiple players typed in the same thing. From that, it could build a massive database of images and corresponding text, which could then be used to register accounts. Alternatively, someone who wants to crack a captcha could submit it to a server, which would then include that captcha in all the currently-running games and reply with the text when several people typed the same thing.