diff --git a/includes/common.inc b/includes/common.inc
index d34c19d..ccb4876 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -6138,6 +6138,14 @@ function element_child($key) {
 /**
  * Return the children of an element, optionally sorted by weight.
  *
+ * By convention, the 'children' of an array are those elements that do not
+ * have a key starting with '#'. This allows a single array to represent a
+ * tree structure where nodes in the tree have properties: The properties of
+ * a node are key/value pairs where the key starts with a '#'. The children
+ * are those key/value pairs where the key does not.  See drupal_render() for
+ * an example how the distinction may be used. element_children() helps
+ * extracting only the children-representing key/value pairs of a given array.
+ *
  * @param $elements
  *   The element to be sorted.
  * @param $sort