diff --git a/lib/one_gadget/gadget.rb b/lib/one_gadget/gadget.rb
index 5bafa79..3426a26 100644
--- a/lib/one_gadget/gadget.rb
+++ b/lib/one_gadget/gadget.rb
@@ -56,6 +56,8 @@ def to_obj
         }
       end
 
+      # To have this class can be serialized in JSON.
+      # @return [String]
       def to_json(*)
         to_obj.to_json
       end