diff --git a/tools/src/main.rs b/tools/src/main.rs
index 9a181f3..73d4bbf 100644
--- a/tools/src/main.rs
+++ b/tools/src/main.rs
@@ -14,6 +14,15 @@ struct Args {
 
 #[derive(Debug, serde::Deserialize)]
 struct Input {
+    /// Each input has either a `quantity` (for discrete things, like
+    /// apples or bearings) or an `amount` (for continuous things,
+    /// like rope or flour).
+    ///
+    /// FIXME: Is there a good way to handle the "units" of the
+    /// quantity/amount? Maybe the units could default to "count", but
+    /// be optionally overridden by the recipe to things like "meters"
+    /// (of tubing) or "grams" (of chromic acid).  Then this struct
+    /// would be `quantity: usize, units: Option<Unit>`.
     quantity: Option<usize>,
     amount: Option<String>,
 }
@@ -36,7 +45,10 @@ struct Action {
 
 #[derive(Debug, serde::Deserialize)]
 struct Recipe {
+    /// `[inputs]` is a Table where each key the the name (unique id)
+    /// of a recipe in the repo, and the value is an Input object.
     inputs: std::collections::HashMap<String, Input>,
+
     dependencies: Dependencies,
     action: Action,
     outputs: Option<Vec<String>>,