When there are two words in a recaptcha, one of which is readable, and the other of which is more-"wtf?", the second word is one which was scanned, and which recaptcha is using your input to generate a croud-sourced "OCR" translation for.

The way this works is, if enough people type the same answer in as the translation for a given text, that is taken as the "real" answer, and will be used in scan-to-text translations of hard to scan books, and as training for OCR systems. ("training" meaning telling an automated system what it should guess as the text when it encounters similar-looking images in the future)

Because the system doesn't actually know the answer for the scanned image (it's asking you for an answer, so it can make a better guess later), you can put whatever you want for that one, and it will be accepted. That's why posts like this sometimes end in "the funniest part is: it was accepted!", you could have typed literally anything in there, as that part isn't actually used to verify who you are, it's just used to tell machines in the future what to tell people the text says if it's ever asked.

To break it down a bit further: say it gives that scan to 100 people. 90 of them say the answer was a random string of characters (because they were spambots, not humans, 9 says "Cobbler," and one says "'ɹǝlqqoɔ". Because "Cobbler," is the answer which people guessed the most, the system can guess that this is the real translation of the scan, and in the future it will lean towards producing that as its real answer, whenever it sees text which looks like that. (It's pretty clever, really, and at this point the accuracy is pretty damn good, which is why "upside-down" and other "truly odd" text is seen more often these days. (they've run out of trivial things that they can't figure out with a high degree of accuracy). This whole "what do people most-often guess?" thing is a pretty good way of finding out what things mean, and has come to be relied on as a very good method of OCR training. The simple concept of "if people guess it, we should use that for future scans" works.

That is why whenever you can tell which part is machine-generated, and which part is a scan, your answer for the scan-part should always be penis.