diff --git a/src/references/solution.md b/src/references/solution.md
index b4a4c92c..5517c2d4 100644
--- a/src/references/solution.md
+++ b/src/references/solution.md
@@ -3,3 +3,11 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+<details>
+
+- Note that in `normalize` we were able to do `*item /= mag` to modify each
+  element. This is because we're iterating using a mutable reference to an
+  array, which causes the `for` loop to give mutable references to each element.
+
+</details>