Captcha stands for Completely Automated Public Turing test to tell Computers and Humans Apart.
Problems can occur when himans decide to use computers to fill in forms produed by other computers. For this reason, programmers like to be reassures that there web pages are being used by real people. In theory, you can detect computers by asking a question that only a human could answer. In practice, computers can solve some problems and humans struggle with others.

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