Captchas are those little tests that prevent automated scripts from registering or submitting content to your site. Usually a weird, skewed image of random numbers or letters that you have to then type into a box. They work on the principal that only a human could decipher this thing. But sometimes the images are so skewed that you can’t read what the darn thing says. Then you have the added inconvenience of typing it in.

This is an example of over-thinking the problem.

Asirra is a "research" project at Microsoft that uses pictures of cats and dogs to tell if you’re human or script. Try it out here [LINK]. Since 2 pictures, one of a cat and one of a dog would give a bot 50/50 chance of getting it right, Asirra has the user click all pictures that are cats (for example) out of a line up of 12 animals. (I had this exact idea which I’d cited in my book (ISBN:0470097779) but with meat and eggs). It’s a little better but still complicated imho. Plus it takes up a tremendous amount of space on the page.

I use a simpler method, just answer the question "what’s my name", if you don’t know then you might be a robot. Now if it ever became a problem I could rotate the question, or re-word it in a way that’d make it difficult for an NLP (Natural Language Processing) enabled script robot to decipher. You’ve got to keep things as simple as possible but no simpler.