Contributors paste code they wrote (a script, a function, a SQL query, a spreadsheet formula) and answer two questions: what it does, and why it was built that way, including the approach rejected. Copied or third-party-licensed code, explanations without the why, and trivial snippets are excluded. One sample per contributor, from a global base strongest in Southeast Asia, the United States and Europe, in whatever languages people work in. Public code is abundant and its reasoning rarely written down: The Stack is huge and permissively licensed, with no record of why a line was written the way it was. The rationale is a decision trace from the author, the supervision reasoning-focused code models lack.