diff --git a/pb/demo.proto b/pb/demo.proto
index 09e5e4d..46e6b7c 100644
--- a/pb/demo.proto
+++ b/pb/demo.proto
@@ -64,6 +64,10 @@ message Product {
     string description = 3;
     string picture = 4;
     Money price_usd = 5;
+
+    // Categories such as "vintage" or "gardening" that can be used to look up
+    // other related products.
+    repeated string categories = 6;
 }
 
 message ListProductsResponse {