diff --git a/extra_capsnets.ipynb b/extra_capsnets.ipynb
index 67e67bd..ea3ae25 100644
--- a/extra_capsnets.ipynb
+++ b/extra_capsnets.ipynb
@@ -32,7 +32,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Watch [this video](https://www.youtube.com/embed/pPN8d0E3900) to understand the key ideas behind Capsule Networks:"
+    "Watch [this video](https://youtu.be/pPN8d0E3900) to understand the key ideas behind Capsule Networks:"
    ]
   },
   {
@@ -42,12 +42,23 @@
    "outputs": [],
    "source": [
     "from IPython.display import HTML\n",
-    "\n",
-    "# Display the video in an iframe:\n",
-    "HTML(\"\"\"<iframe width=\"560\" height=\"315\"\n",
-    "                src=\"https://www.youtube.com/embed/pPN8d0E3900\"\n",
-    "                frameborder=\"0\"\n",
-    "                allowfullscreen></iframe>\"\"\")"
+    "HTML(\"\"\"<iframe width=\"560\" height=\"315\" src=\"https://www.youtube.com/embed/pPN8d0E3900\" frameborder=\"0\" allowfullscreen></iframe>\"\"\")"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "You may also want to watch [this video](https://youtu.be/2Kawrd5szHE), which presents the main difficulties in this notebook:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "HTML(\"\"\"<iframe width=\"560\" height=\"315\" src=\"https://www.youtube.com/embed/2Kawrd5szHE\" frameborder=\"0\" allowfullscreen></iframe>\"\"\")"
    ]
   },
   {
@@ -510,6 +521,7 @@
    "metadata": {},
    "source": [
     "We can apply this function to compute $\\hat{\\mathbf{u}}_{j|i}$ for every pair of capsules ($i$, $j$) like this (recall that there are 6×6×32=1152 capsules in the first layer, and 10 in the second layer):\n",
+    "\n",
     "$\n",
     "\\pmatrix{\n",
     "  \\mathbf{W}_{1,1} & \\mathbf{W}_{1,2} & \\cdots & \\mathbf{W}_{1,10} \\\\\n",
@@ -2172,7 +2184,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.5.2"
+   "version": "3.6.3"
   }
  },
  "nbformat": 4,