Signing up for a webmail account or social networking site usually requires the user to decipher and re-type some scrambled letters first. These text systems are called Captchas (Completely Automated Public Turing Test to tell Computers and Humans Apart). Users are asked to fill out a captcha form to confirm they are a real person rather than a computer program. Their purpose is to stop hackers creating programs that could automatically sign for thousands of accounts. Scammers and spammers would then exploit these to send out more effective junk mail.

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