diff --git a/crates/fj-kernel/src/objects/solid.rs b/crates/fj-kernel/src/objects/solid.rs
index 65c53533c..74982135e 100644
--- a/crates/fj-kernel/src/objects/solid.rs
+++ b/crates/fj-kernel/src/objects/solid.rs
@@ -12,6 +12,12 @@ use super::{Face, Surface};
 ///
 /// The faces that make up the solid must form a closed shape. This is not
 /// currently validated.
+///
+/// In fact, solids could be made up of several closed shells. One outer shell,
+/// and multiple inner ones (cavities within the solid). There should probably
+/// a separate `Shell` object that is a collection of faces, and validates that
+/// those faces form a closed shape. `Solid` should be a collection of such
+/// `Shell`s, and validate that those `Shell`s don't intersect.
 #[derive(Clone, Debug, Eq, PartialEq, Hash, Ord, PartialOrd)]
 pub struct Solid {
     faces: BTreeSet<Face>,