We were talking about this project at… oh nevermind… just check out this awe-inspiring 4-bit computer built entirely out of K’Nex. Here’s the description from the site:
“The K’NEX calculator stands over 10 feet tall, and can perform 4 bit addition and subtraction operations in about 30 seconds. The slowest part of the operation is the user entering the balls. From there the balls trickle down, computing the result of the operation, and then sending that through a 4 bit decoder, which flips a flag that tells the user the answer. Since it is 4 bit, we can add and subtract numbers from 0 to 15.”
Read the Ball Theory page for a succinct and graphical description of how this system can be used to create the various logic gates needed in computing (and some of the design challenges involved). The videos explain a lot, too.