I'm also a member of the "Taught my wife binary just for fun" club, but I used fingers in a different way than Rick Garlikov did. I told her that there was a way to count to 512 using just your fingers, and let her work through how it might be possible using a similar question-answer trial and error approach. Once she realized that each finger had 2 states (up or down), she naturally recreated a binary counting system. It's a neat trick, and even comes in handy when trying to keep track of a large count of something. With kids, however, you may end up with behavior problems around the numbers 4 and 132 :)