@@ 1264,7 1264,10 @@ deployment}. When a pre-built binary for a @file{/gnu/store} item is
available from an external source---a @dfn{substitute}, Guix just
downloads it and unpacks it;
otherwise, it builds the package from source, locally
-(@pxref{Substitutes}).
+(@pxref{Substitutes}). Because build results are usually bit-for-bit
+reproducible, users do not have to trust servers that provide
+substitutes: they can force a local build and @emph{challenge} providers
+(@pxref{Invoking guix challenge}).
Control over the build environment is a feature that is also useful for
developers. The @command{guix environment} command allows developers of