CAPTCHAs are challenge questions employed by websites to prevent access by automated programs (bots) while allowing access to human users. The effectiveness of a CAPTCHA in distinguishing between humans and bots is sometimes reduced, because as bots get more powerful, they are able to solve certain types of challenge questions almost as well as humans. This disclosure describes CAPTCHAs based upon word relationships. Solving the CAPTCHAs requires demonstrating natural language understanding. Automated programs are not yet comparable to humans in their ability to understand natural language. CAPTCHAs of this disclosure provide useful techniques to distinguish human users from bots.

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This work is licensed under a Creative Commons Attribution 4.0 License.