1. TechTarget: CAPTCHA (Completely automated public turing test to tell computers and humans apart),
https://searchsecurity.techtarget.com/definition/CAPTCHA
2. Word Press: CAPTCHA code authentication,
https://wordpress.org/plugins/captcha-code-authentication
3. The Register: Facebook serves up shaved tattooed ‘butterfly’ as CAPTCHA,
https://www.theregister.co.uk/2015/04/30/facebook_serves_up_shaved_pierced_tattooed_butterfly_as_captcha
4. Geek: Gamified CAPTCHAs dispense with distorted text,
https://www.geek.com/games/gamified-captchas-dispense-with-distorted-text-1573300
5. Cui J et al (2010) CAPTCHA design based on moving object recognition problem. In: The 3rd international conference on information sciences and interaction sciences, Chengdu, pp 158–162