If I knew any woodworking, I would absolutely make myself one of these beautiful binary marble adding machines. It’s a six-bit binary adder, implemented with marbles and wooden toggles that allow for carrying. The page describes in a good bit of detail how it works, but if you just want the high-level overview, scroll to the bottom and watch the video demonstration. It’s really pretty amazing.
Glimpses of the websurfing habits of a computer science professor, obsessive crafter, occasional gamer, and all around geek