diff --git a/cookbook/introduction/22_Provenance_Tracking.ipynb b/cookbook/introduction/22_Provenance_Tracking.ipynb
index 7df9ab1d..de54ddf1 100644
--- a/cookbook/introduction/22_Provenance_Tracking.ipynb
+++ b/cookbook/introduction/22_Provenance_Tracking.ipynb
@@ -91,7 +91,9 @@
    "source": [
     "## Step 2: Track the Relationship Between Facts\n",
     "\n",
-    "Facts rarely stand alone. The claim about biomass increase is *about* the marine reserve — that relationship is a first-class provenance-tracked object too."
+    "Facts rarely stand alone. The claim about biomass increase is *about* the marine reserve — that relationship is a first-class provenance-tracked object too.\n",
+    "\n",
+    "`track_relationship()` has no dedicated subject/object fields, so by convention we record which two entities it connects inside `metadata`."
    ]
   },
   {
@@ -103,10 +105,16 @@
     "rel = prov.track_relationship(\n",
     "    relationship_id=\"rel_biomass_about_reserve\",\n",
     "    source=\"DOI:10.1371/journal.pone.0023601\",\n",
-    "    metadata={\"type\": \"measured_at\"},\n",
+    "    metadata={\n",
+    "        \"type\": \"measured_at\",\n",
+    "        # No dedicated endpoint fields on track_relationship() yet -- record\n",
+    "        # which entities this relationship connects here by convention.\n",
+    "        \"subject_entity_id\": \"claim_biomass_increase\",\n",
+    "        \"object_entity_id\": \"marine_reserve_1\",\n",
+    "    },\n",
     ")\n",
     "\n",
-    "print(\"Relationship tracked:\", rel.entity_id)"
+    "print(\"Relationship tracked:\", rel.entity_id, \"|\", rel.metadata[\"subject_entity_id\"], \"->\", rel.metadata[\"object_entity_id\"])"
    ]
   },
   {