diff --git a/FAQ.md b/FAQ.md
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@@ -27,6 +27,11 @@ might add:
 
 to your `.ccls` file.
 
+## When does ccls reindex a file?
+
+* The timestamp of the file, or any of its dependency has changed. See `pipeline.cc:indexer_Parse`.
+* The clang command line options have changed. See  `pipeline.cc:cacheInvalid`.
+
 ## Full indexing takes a long time / crushes my system
 
 For large projects, full indexing can be a significant burden.  Once the