To prove you are a human and not a robot, internet sites and blogs use the CAPTCHA. Enter the two words into the box to prove you have eyes and not a robot who is more interested in leaving spam.
The story about the CAPTCHA is at TED. CAPTCHA creator Luis von Ahn describes in massive-scale online collaboration how you help in creating digitized books from scanned books. As you enter the characters of the word, you assist in making sense of words that may not have scanned as a recognizable word.
Now doesn't that make you feel good knowing that you are contributing some small effort to digitizing old books.
TED is a destination for educational, entertaining and thinking pieces which are rarely found on the internet. The site is checked about once a month to see the latest posted talks. It is one of those few sites where I don't mind eating into my monthly internet limit of 5GB per month.
Even though CAPTCHAs are used on my blog for comments, there has been the very infrequent spam comment. Those spam comments will include a URL to sell some product. When found, the comment is deleted. Don't know how they got there. A robot computer program. Slave labor in India or China. Don't know. If the spam became a problem, comments would be turned off.