>>27I've actually been thinking about that sort of thing as a captcha system. The biggest problem with the ``How many fingers does a human hand have?'' --> ``5'' system is that somebody has to write all the questions to begin with, which limits the size of the question database, and all a bypasser has to do is build up their own counter-database. But if the questions were distorted before being presented, it would make figuring out what the original question was quite difficult, while a human would be able to pretty easily answer the question.
Another approach would be to make some sort of attempt to automatically generate questions, and then have humans fill out two as in ReCAPTCHA, one known, the other generated. If a generated question gets back a bunch of conflicting/nonsense answers, the system can throw it away, but if the question consistently gets back ``5'', it can be added to the list of known good questions.