Here's a teaching tool. It's a binary marble adding machine. Its based on the plans from the site: woodgears.ca/marbleadd/. It's a great way of teaching binary numbers and it is also a lot of fun to watch the marbles fall!
This is designed to be laser cut in 5mm (1/4" nominal) plywood and then all glued 3 thick for the bars, and the rocker mechanisms.