A Captcha is used to identify the user of a form, for example one to post comments in a Blog, as human. It s a widespread technique to use Visual Captchas, in which the person has to read a code out of an image and write it into one form field. This works, because the human mind is very good at recognizing written things, at which the computer is pretty bad. But Visual Captchas have been solved by programmers, as one can read here.

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