diff --git a/mercure.rst b/mercure.rst
index f37c40ddee7..698de373a17 100644
--- a/mercure.rst
+++ b/mercure.rst
@@ -309,6 +309,11 @@ as patterns:
     }
     </script>
 
+However, on the client side (i.e. in JavaScript's ``EventSource``), there is no built-in way
+to see which topic a certain message is coming from. So if this (or any other meta information)
+is important to you, you need to include it in the message's data (e.g. by adding a key to the
+JSON, or a `data-*` attribute to the HTML).
+
 .. tip::
 
     Test if a URI Template matches a URL using `the online debugger`_