reCAHTCHA

http://recaptcha.net/learnmore.html - 之前一直不知道reCAHTCHA的原理，簡單的說，分成兩個部分，第一個部分是知道正確答案的word(s)，用來確認，第二部分是書 scan 出來的圖，統計最多人的就當作正確答案。不過如果知道上下文的話，人的辨視度會高很多。

http://recaptcha.net/learnmore.html
But if a computer can't read such a CAPTCHA, how does the system know the correct answer to the puzzle? Here's how: Each new word that cannot be read correctly by OCR is given to a user in conjunction with another word for which the answer is already known. The user is then asked to read both words. If they solve the one for which the answer is known, the system assumes their answer is correct for the new one. The system then gives the new image to a number of other people to determine, with higher confidence, whether the original answer was correct.