>      I tried to cram all of the proposed mechanisms into one example
> (all except the spelling errors, which I hadn't thought of yet); I see
> now that this was a very bad way to present them.  Imagine instead of my
> duck problem a whole collection of questions that require different
> sorts of answers but look structurally similar to some of the other
> questions in the collection.  In other words:
> 
>       * If I have seven pairs of shoes how many shoes do I have?
> 
>       * What goes "quack"?
...etc

I can only think of a few different ways to generate the pool of questions
to show to the user and the expected answers:

1. Someone writes a database of questions and answers and distributes it
   to whoever wants it (but then the spammers get it as well)

2. Someone writes a knowledgebase which can be used to generate the
   questions algorithmically (but that means it will be easy to reverse
   the process, given the same knowledgebase and the program which generates
   the questions)

So you would also need some obfuscation: enough to be hard to reverse for a
computer program, but not enough to confuse a human.

Alternatively:

3. Each wiki author writes their own questions and answers (too much work)

4. Each visitor to the site can contribute a new question and answer
   (risks pollution with questions that don't work; probably needs
   moderating)

Others?

Regards,

Brian.