Sploid reader Kimberly Nguyen has spotted this really neat video demonstrating how computer logic gates work using just strings and weights. "I'm an Electrical Engineering/Computer Engineering major and I've never seen anything like this before!" Me neither. It's really clever.
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.