> If we figure that the foil can meaningfully change direction every 20 μm, then we might think of an aluminum-foil machine as being made of “moving parts” on the order of 1000 μm² (50 μm × 20 μm), 1000 “parts” per square millimeter of foil; a roll of kitchen aluminum foil is enough to fabricate some 4 billion “parts”. A bootstrapping compiler might require 100 000 parts and thus a square centimeter of aluminum foil, cut and folded around into a shape a couple of millimeters in diameter. If it were doing only one thing at a time, and needed 10 seconds to construct/assemble each moving part, it would take about 12 days to recompile itself. This is probably adequately fast, barely, but probably not adequately robust against errors. It would probably be better to design it to have more parts and do many things at once, enabling it to be faster and correct errors.