motorhaven - 3:55 am on May 24, 2012 (gmt 0)
Another method is displaying a random picture of something and has a limited number of answers about what it is. For instance asking the user to type in what kind of animal a picture of a dog is. Possible answers could be dog, canine, bread of the dog (and close matches), etc. Have a couple of dozen of these and add a new one to the mix every few months... works great.