You know those annoying things with a distorted word or series of letters they make you type before you can sign up for some sites?
This site uses words scanned in from books which the automated software was unable to read, along with one word which the system has recognised. If you get the word it does know correct, it assumes you’re right for the other word too.
Thus, that few seconds of otherwise wasted human brain-power has gone towards ensuring a book somewhere is correctly digitised. Genius!