When you go online and have to determine a captcha word, it's likely that word is from an old text that's been digitized.
When old texts are digitized, we've got technology to make an image of the text, encode the text so it can be searched. But, we don't have technology that can do well at correcting mistakes. That's where humans come in (so glad we're still good for something!). A researcher at Carnegie Mellon University, Dr. von Ahn "estimated that humans around the world decode at least 200 million Captchas per day, at 10 seconds per Captcha. This works out to about 500,000 hours per day -- a lot of applied brainpower being spent on what Dr. von Ahn regards as a fundamentally mindless exercise. "

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