diff --git a/guide/under-the-hood.md b/guide/under-the-hood.md
index 43b9830..5fc2043 100644
--- a/guide/under-the-hood.md
+++ b/guide/under-the-hood.md
@@ -156,7 +156,7 @@ end
 
 Let’s go back to our `HelloWorld` component and add a class attribute to each of the divs.
 
-```ruby
+```ruby{3-4}
 class HelloWorld < Component
   def view_template
     div(class: "outer") {