Junk e-mail operators use programs called bots to register for thousands of free online e-mail accounts. Bot-blocking obstacles offer some protection; Yahoo!'s e-mail sign-up form, for example, asks users to type in an English word that's displayed in the form of a spotty, degraded image that humans can decipher but bots cannot (see "Excuse Me, Are You Human?"). Trouble is, hackers can build bots that bust such barricades by matching the outline of the word in the degraded image with the outlines of words in a dictionary.

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