The essence of digital computing is the use of continuous physical states to represent a discrete number of symbols and the ability to perform logic based on those symbols. Although electronic circuits are exceptionally well-suited for this, any system that can handle symbols as both input and output is a digital computer. Here, I've demonstrated the construction of simple digital computers (specifically, binary logic gates) using pulleys and weights.
Want to build your own?
See the stuff you'll need here: http://bit.ly/pulleylogicgatesstuff
See pics here: http://bit.ly/pulleylogicgatespics
And feel free to reach out to me with questions: firstname.lastname@example.org