They are called "CAPTCHAs". Now you know when there are two words. Here's what's going on - you're helping to digitize some old books or newspapers.

"NEIL DeGRASSE TYSON: Luis's solution was to take these hard to read words from old books and use them as CAPTCHAs.

"But this raised a new problem, the computer would now present a word that it could not read in the first place.

"MANUEL BLUM: The computer didn't know what the answer was. How is it to be able to tell what the right answer is?

"NEIL DeGRASSE TYSON: Luis found a solution: to combine the word from an old book with a traditional CAPTCHA generated by the computer.

"MANUEL BLUM: We'll give two tests, one that we know the answer to, one that we don't. And if the person can solve the one that we know the answer to, then we'll assume they can solve the one that we don't.

"NEIL DeGRASSE TYSON: They called it reCAPTCHA. Now, every time you type a CAPTCHA, you may very well be working for Luis, transcribing an old book.

"PETER LEE: Today, on the order of 125 to 150 books per day are being digitized because of reCAPTCHA. It's an amazing thing. "