Ask HN: What ideas do you have for creating the ideal CAPTCHA?

CAPTCHA - Completely Automated Public Turing test to tell Computers and Humans Apart

An ideal CAPTCHA would have the following:

   1. Human Verification - Only be able to completed by humans
   2. Additional Use     - Example: reCAPTCHA

Human verification has to include something that can be done easily and universally by all humans (including disabled people), but be impossible for any machine to do. An additional use is simply a bonus, as it converts the time spent on the CAPTCHA into something useful.

We have a lot of different CAPTCHA's out there already and the most common one is reCAPTCHA. However, we should think outside the box in order to leverage other skill sets that we have that computers don't.

Personally, I find this area very interesting with many possibilities. What do you think?

Examples:

   http://www.google.com/recaptcha
   http://blogoscoped.com/archive/2010-05-03-n58.html
   http://www.arraystudio.com/as-workshop/the-captcha-alternatives.html
   http://www.evengrounds.com/developers/alternatives-to-captcha
   http://onlineaspect.com/2010/07/02/why-you-should-never-use-a-captcha/