I believe they cross-check between different users. I believe it used to work something like this on the old recaptcha system (the one with the words or house numbers): They show you 2. 1 is known by recaptcha, the other one isn't. If you enter the known one correctly, you can enter. The unknown one is presented to other users and when there is enough consensus amonst users it is promoted to a known one. So it's hard to mess with the system as an individual.