CAPTCHA.

Do you ever get sick of having to enter words at the botton of your comments to prove your not a robot? You are doing a little job for the world by entering those words, your saving old books! 

It's called CAPTCHA. People scan words from the old books and put them at the end of you comments or posts to prevent spam and also to help them find out the words that are in the books that others can't read. They would use a computer to convert them but the computers are not as smart as us humans so they use us to translate them. They are translating around 2 hundred millon words a day it's surprising how big the internet is.