Have you ever wondered how a spreadsheet works? I was fairly #mindblown when someone first explained it to me on the back of a beermat -- it's simpler than I thought! Let me take you on a tour of the source code for Project Dirigible, an experimental web port of Resolver One, the (sadly defunct) Pythonic spreadsheet. We'll cover formula parsing, identifying dependencies, building the recalculation graph, loop detection, and even some of the mind-bending fun that Dirigible lets you do with user-defined functions.