BBC - Digital Revolution Blog: Recycling clicks to benefit humanity
For example, CAPTCHAs prevent scalpers from writing programs to buy millions of tickets for concerts or sporting events. It is estimated that over 200 million CAPTCHAs are typed every day, each taking roughly ten seconds of human effort - that's 500,000 hours a day. ReCAPTCHA re-cycles this human mental effort into a dual purpose: transcribing books.

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