In 1936, Alan Turing wrote a landmark paper about what kinds of numbers could be computed: On Computable Numbers. In it, he described what's now known as a Turing machine, which computes with a tape it can scroll back and forth, reading and writing ones and zeros, and transitioning among internal states based on a program.
It was only a thought experiment, a hypothetical machine meant to explore the possibilities of an ultra-simple computer. That is, it was, until Mike Davey built an actual Turing machine. Not an electronic one, but an actual mechanical device that reads and writes ones and zeros on a scrolling tape:
It's a great piece of work, complete with a single-step debugger. The craftsmanship and dedication to the original description are remarkable. And it's amazing to see the juxtaposition of the spool of primitive tape, being driven by an SD card which is commonplace now, but has millions of times the capacity and accessibility.