Over the coming weeks, I'll highlight a few of the best (and weirdest) CAPTCHA systems available on the web. CAPTCHAs are those "type this word" or "answer this question" tests you see on many web forms -- they're there in an attempt to prove that the entity filling out the form is not a spambot.

The term CAPTCHA is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart." (The subject of Turing tests deserves its own blog entry, but let's just summarize here by saying that such tests are designed to test human-style intelligence.) While most CAPTCHA systems are designed using scrambled text -- something that's easy for humans to decipher but hard for computers -- there are some interesting variants in the wild. Let's start with my favorite weird CAPTCHA system: KittenAuth.

Developed by programmer Oli Warner, the KittenAuth system presents a series of pictures of cute animals, and asks the user to click on all the kittens. Well, that's the simplest form -- the current version may ask you to pick out a different animal, so you may have to click on all the pandas or lambs -- this adds some fun to the game, and presumably prevents spammers from investing all their effort on kitten-detection software. Here's a screenshot of an example from Warner's contact form (it's not clickable):

I like KittenAuth because it's cute and actually kind of fun. I wouldn't want to use the system constantly (for example, every time I added a Facebook friend), but for occasional use it's a great idea -- and surprisingly hard for a spambot to crack. It's difficult for a computer because image recognition is both difficult and computationally expensive.

In future weeks I'll go in-depth on the ReCAPTCHA system used on this very blog, and other interesting variants. But in the meantime, we've noticed a lot of commenters finding bizarre and interesting things in the ReCAPTCHA boxes. For example, today adrienne wrote: "I love ReCaptcha: 'Philbin girl' this time." We even held a CAPTCHA Contest in April in which commenters made poetry out of the CAPTCHA text. But I thought I'd ask: what's the craziest CAPTCHA text you've seen? I just tried two at random, and got 'integrated ex' and 'tingle tempers.' This is art, people.