Captcha stands for Completely Automated Public Turing test to tell Computers and Humans Apart.
Problems can occur when computers try to complete forms which are designed for humans. For this reason, programmers sometimes need to verify 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 be trained to answer many of these questions and humans often fail these tests.

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