diff --git a/README.md b/README.md
index 7e10dc1..aa840c1 100644
--- a/README.md
+++ b/README.md
@@ -104,3 +104,5 @@ Saving an Activity to this table may have side-effects, based
 on the Activity type. For example, saving a Follow activity
 constitutes a Follow request.
 
+Activities have a "valid" flag, because they can be undone
+by subsequent Activities of type Undo.