Sign up for an account on almost any website and you'll be asked to read and type out a string of distorted characters. This task, known in the business as a Captcha, is meant to be easy for a human but near impossible for a software "bot," thus preventing spammers from automatically creating accounts in order to send out e-mail or social networking spam.

A startup called NuCaptcha plans to refresh the Captcha concept by asking users to enter text displayed as part of a short video advertisement. The service launches today with advertisers including Disney and the computer games publisher Activision.

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