Another test that is hard for computers but easy for us is image recognition - this is where flickr comes in. By selecting certain random images from flickrs huge base we can create a test that we can use to authorise a person.
It's quite a straight forward process. We just need two different sets of photos, mix them up and allow a user to choose one of the sets. In flickr we can get a selection of photos by tag name. We connect to flickr and request a number of photos with two different tags - the tags should be contained in two unassociated categories: "tiger" and "lion" would not be a good choice, but something like "banana" and "zebra", or "spaghetti" and "clouds" are better.
Here's an example using "giraffe" and "moon". There is currently a total of 268166 and 1364515 respectively.
Select the photos tagged with "giraffe".
This shows how the basic system would work. Obviously you'd have to have a back end system to do the checking but this should give you a good idea of how it would work.
You can try some other words by typing in the boxes below. You can try a group of words by seperating them with commas.