Part of our job as package maintainers in various distributions is to ensurethe binary packages can be built from the corresponding source code. This isoften a requirement, either from a distribution's own policies (eg, the DFSGfor Debian), or from the license of the upstream package (eg, the GPL). Butalthough we are used to seeing .c and .py files as source, we sometimes forgetthat our packages are built from many other files for which we also must makethe source available. In this talk I will explain why we require source code in the first place;without the availability of source code, the whole free software ecosystemwould not exist. I will also describe file types that we sometimes forget tosee as source, or which are considered binary but we forget to include thesource code for. This includes documentation, build scripts, files generatedat build time by various programs, fonts, multimedia such as sound, music,images and even video. I will also describe various problems we can encounterwhen trying to determine what the actual source is, and that even if we knowwhat the source is, that it is sometimes creates practical problems fordistributions to include all of it.