diff --git a/java/src/com/android/inputmethod/research/ResearchLogger.java b/java/src/com/android/inputmethod/research/ResearchLogger.java
index f4249a045..79a11fbaf 100644
--- a/java/src/com/android/inputmethod/research/ResearchLogger.java
+++ b/java/src/com/android/inputmethod/research/ResearchLogger.java
@@ -852,6 +852,11 @@ public class ResearchLogger implements SharedPreferences.OnSharedPreferenceChang
         mCurrentLogUnit = newLogUnit;
     }
 
+    /**
+     * Record the time of a MotionEvent.ACTION_DOWN.
+     *
+     * Warning: Not thread safe.  Only call from the main thread.
+     */
     private void setSavedDownEventTime(final long time) {
         mSavedDownEventTime = time;
     }