Mike Davey wanted to build a real Turing machine, but unfortunately he could not find the infinitely long tape required for the project. His solution? Using 300m of white 35mm film leader and a dry erase marker. Result? Brilliance.
Davey's description of the machine almost makes your head spin a tiny bit, but after watching the video above, it'll make far more sense:
Although this Turing machine is controlled by a Parallax Propeller microcontroller, its operation while running is based only on a set of state transformations loaded from an SD card and what is written to and read from the tape. While it may seem as if the tape is merely the input and output of the machine, it is not! Nor is the tape just the memory of the machine. In a way the tape is the computer. As the symbols on the tape are manipulated by simple rules, the computing happens. The output is really more of an artifact of the machine using the tape as the computer.