On autoki.com we have this “Tell-a-friend” functionality, where people can enter the email addresses of their friends and have the link to a cool photo or car sent to them. Until recently this was only accessible to logged in users because we were afraid of spam bots using it to send emails to everyone using our servers. At some point we realized that this function could be much more useful if everyone would be able to use it - member or not. The solution we chose to fight the spam bots was pretty standard: Captchas - Completely Automated Public Turing tests to tell Computers and Humans Apart. (I just love this name)

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