Everything You Ever Wanted To Know About CAPTCHAs But Were Afraid To Ask

Love them or hate them - CAPTCHAs have become ubiquitous on the Internet.

What on earth is a CAPTCHA anyway, and where did it come from? Responsible for eye-strain the world over, the humble CAPTCHA has been the centre of much attention as the single most effective weapon in the fight against web spam. But are they effective?

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http://www.makeuseof.com/tag/wanted-captchas-afraid-technology-explained/