diff --git a/lib/cpp/src/concurrency/Mutex.h b/lib/cpp/src/concurrency/Mutex.h
index 884412b..73c73e0 100644
--- a/lib/cpp/src/concurrency/Mutex.h
+++ b/lib/cpp/src/concurrency/Mutex.h
@@ -102,6 +102,8 @@
 
 
 // A little hack to prevent someone from trying to do "Guard(m);"
+// Such a use is invalid because the temporary Guard object is
+// destoryed at the end of the line, releasing the lock.
 // Sorry for polluting the global namespace, but I think it's worth it.
 #define Guard(m) incorrect_use_of_Guard(m)
 #define RWGuard(m) incorrect_use_of_RWGuard(m)