diff --git a/04_mnist_basics.ipynb b/04_mnist_basics.ipynb
index 14745de..d6aeaf6 100644
--- a/04_mnist_basics.ipynb
+++ b/04_mnist_basics.ipynb
@@ -3887,7 +3887,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Now let's see what the change in accuracy is for a small change in one of the weights:"
+    "Now let's see what the change in accuracy is for a small change in one of the weights (note that we have to ask PyTorch not to calculate gradients as we do this, which is what `with torch.no_grad()` is doing here):"
    ]
   },
   {
@@ -3896,7 +3896,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "weights[0] *= 1.0001"
+    "with torch.no_grad(): weights[0] *= 1.0001"
    ]
   },
   {