diff --git a/advanced_topics/object_oriented_programming.ipynb b/advanced_topics/object_oriented_programming.ipynb
index 3edc786ead174a24360055e63a2c7e2e28a0c638..6ce41605fc589428e8462bc0d9ba33ea93930497 100644
--- a/advanced_topics/object_oriented_programming.ipynb
+++ b/advanced_topics/object_oriented_programming.ipynb
@@ -1281,6 +1281,10 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
+    "> Simple classes such as the `Notifier` are sometimes referred to as\n",
+    "  [_mixins_](https://en.wikipedia.org/wiki/Mixin).\n",
+    "\n",
+    "\n",
     "If you wish to use multiple inheritance in your design, it is important to be\n",
     "aware of the mechanism that Python uses to determine how base class methods\n",
     "are called (and which base class method will be called, in the case of naming\n",
diff --git a/advanced_topics/object_oriented_programming.md b/advanced_topics/object_oriented_programming.md
index 68be0603739e08005ba38acfc30fd3fd47dcd540..e5504cf616955e1f41545526f339e15f22abb1ac 100644
--- a/advanced_topics/object_oriented_programming.md
+++ b/advanced_topics/object_oriented_programming.md
@@ -1024,6 +1024,11 @@ so.do('concat', '?')
 print(so.run('did you notice that functions are objects too'))
 ```
 
+
+> Simple classes such as the `Notifier` are sometimes referred to as
+  [_mixins_](https://en.wikipedia.org/wiki/Mixin).
+
+
 If you wish to use multiple inheritance in your design, it is important to be
 aware of the mechanism that Python uses to determine how base class methods
 are called (and which base class method will be called, in the case of naming