Have you ever signed up for an internet forum or web app? Chances are, you’ve seen a CAPTCHA: a little image with distorted letters demanding that you prove that you are human. Or is that what it is really asking? Perhaps instead, it’s asking that you prove that you’re sighted.
CAPTCHAs (Completely Automated Public Turing test to tell Computers and Humans Apart), are supposed to prevent automated computer programs from posting spam messages on public forums. It’s a kind of Turing test that assumes that computers cannot pass.