It shouldn't be about being able to "read". Ideally it should be something that only human could "solve". May be stupid example, but I can't come up with a better one (and if I could, I would be a lot wealthier by now ;) ): show a picture where a forest in landscape is violet and ask a user to identify what's odd on that picture. Another example, show a face with two noses. Ask them to identify the odd item. Things like that. Easy for human, impossible for computers. The problem is that these tasks need to be generated by humans, as I cannot think of any (irreversable) way to do this automatically. May be mechanical turk to the rescue?