diff --git a/README.md b/README.md
index 79d0e5e..b8ce24c 100644
--- a/README.md
+++ b/README.md
@@ -11,4 +11,9 @@ The fires sometimes disappear from view due to cloud cover.
 The code combines the two satellites' data for increased resolution.
 It currently measures the areas of hotspots. 
 If I have more time I might try doing some other measurements,
-but I also wrote this to demonstrate the accessibility of citizen science.
+but I also wrote in a hurry this to demonstrate the accessibility of citizen science.
+
+If you want to play with the code online,
+one way is to import this repository onto [Google Colab](https://colab.research.google.com/).
+Change the parameters of `download.py` to import new data,
+and perform the analysis with `analysis.ipynb`.
diff --git a/analysis.ipynb b/analysis.ipynb
index e6e4329..6cb5745 100644
--- a/analysis.ipynb
+++ b/analysis.ipynb
@@ -179,7 +179,7 @@
     "        # the y coordinate is flipped\n",
     "        mask = (X_raw - cx)**2 + (Y_raw + cy)**2 < fire[\"r\"]**2\n",
     "        \n",
-    "        fire[\"area\"] = np.sum(data[mask[0]] > 0.1) * (km_per_pixel**2)\n",
+    "        fire[\"area\"] = np.sum(data[mask] > 0.1) * (km_per_pixel**2)\n",
     "        \n",
     "        patch = plt.Circle(\n",
     "            fire[\"offset\"], fire[\"r\"], color=fire[\"color\"], \n",