Well I thought hours in a day was pretty much a standard. The point either way was that the riddles could be constructed to either be 'known' or look up able. Its not an ideal solutions, i'm not sure there is an idea solution, but if you want to make sure its a person on the other end then make them do something people normaly do better than computers. In general people will be better at finding/knowing answers to a mass of questions. Also you could have three sets of riddles and the person could only answer 2 and still pass, or you could have a set of region dependent riddles, letting the user select his/her region before hand.
How many planets are occupied by humans + how many moons does earth have?
how many days are in a week + how many days are in a weekend?
brides in a wedding + eggs in a dozen + sodas in a six pack
WOW! Thanks for the heads up, that could have bitten me in the butt hard... I had considered that for a system I am working on, guess that one goes off the drawing board now :)
Just goes to show, no matter how much you think out a solution, there is always something lurking around the corner that you just don't expect.