If you want to leave a comment on this post, or on many other sites that accepts user input, you'll first have to type in a couple of deformed looking words like this

This is called a reCAPTCHA and it's there to make sure you're human and not some spam-crazed 'bot' looking to fill up our pages with hundreds of adverts for knock-off trainers and hooky medicines.

But by typing in these words you're not only helping to frustrate spammers...

You're also contributing to the Google Books project - Google have already scanned over 12 million books which are available (copyright permitting) to read online. However the scanning process isn't perfect and certain words can't be recognised automatically. The words you're asked to type in are examples of such words that the scanning process can't make sense of but that generally we can read quite easily. When enough people have typed in the same answer for the same unknown word, that answer is taken by the Google Books software as being the correct value.

In effect, it's a kind of distributed proof-reading system that will help perfect a project which 'has the potential to offer unprecedented access to what may become the largest online corpus of human knowledge'.

So, post a comment and chip in!

Detailed info here