The way it works is that the one legible word is the control string, and the other one is the challenge string -- in cases like this it's obvious which is which

The way it works is that the one legible word is the control string, and the other one is the challenge string -- in cases like this it's obvious which is which, but not always. The challenge string could be comprised of characters from different scans, each of which had failed recognition by OCR software. The control word is there to prove that you're a human, and the challenge word is there for you to provide a small amount of work. In this case the work could benefit different scans at one time.