What's wrong with asking easy questions? Have spambots beaten that system. 2+2=, what color is this?, what animal is shown?...
That's a really good question. They have a few weaknesses.
1. Math problem can be easily solved by a computer.
Go to
www.google.com and type 2+2= and you'll see what I mean.
If you
really want to see something amazing, go to
http://www.wolframalpha.com and type in x^2+(y-(x^2)^(1/3))^2 = 1 and watch it solve it.
2. Not everyone speaks English
"What animal is show?" is language dependant.
3. Some culture actually associate negative feeling with some animals.
e.g. An owl in the US is wisdom, but in other cultures it's an omen of death.
4. Any image based captcha system would also have to have an unlimited supply of images. Most captcha systems do not hand out the same captcha more than a few times.
Fun example: I selected a random image from an RSS feed, so it's fresh. I used the Chrome Extentions, "Image Search" from google and searched for it.
Here's what it found.
Not only did it find all references to the
original image, it guessed what search would turn up that image!
