Microsoft Research has developed Asirra, a CAPTCHA system that uses something far more reliable than scrambled letters: Pets! The system presents the user with twelve random photos of cats and dogs pulled from Petfinder.com’s database and asks the user to click on the cats, and only the cats. The idea is that only a human, but not a bot, could successfully identify the cats.
It’s a great idea, and easier to understand than some CAPTCHAs (some of which are just awful), but it suffers from a simple problem: You can’t always tell if a cat is not a dog. Some cats, particularly dark ones, are hard to distinguish from dogs, and I even found one photo that had a cat sitting next to a dog! Still, even though I failed twice and my wife failed once, it seems almost impossible that a computer would ever pass it, except through random chance.
You can use Asirra on your website, and Microsoft gives the full instructions. And if you were wondering, Asirra stands for:
Animal Species Image Recognition for Restricting Access
(via the Virtual Earth blog)