diff --git a/rmarkdown.Rmd b/rmarkdown.Rmd
index 60615f6..085becf 100644
--- a/rmarkdown.Rmd
+++ b/rmarkdown.Rmd
@@ -291,9 +291,16 @@ Caching the `processed_data` chunk means that it will get re-run if the dplyr pi
 
 `dependson` should contain a character vector of *every* chunk that the cached chunk depends on. Knitr will update the results for the cached chunk whenever it detects that one of its dependencies have changed.
 
-Note that the chunks won't update if `a_very_large_file.csv` changes, because knitr caching only tracks changes within the `.Rmd` file. That means it's a good idea to periodically clear out all your caches and start from scratch by running `knitr::clean_cache()`.
+Note that the chunks won't update if `a_very_large_file.csv` changes, because knitr caching only tracks changes within the `.Rmd` file. If you want to also track changes to that file you can use the `cache.extra` option. This is an arbitrary R expression that will invalidate the cache whenever it changes. A good function to use is `file.info()`: it returns a bunch of information about the file including when it was last modified. Then you can write:
 
-Note that I've used the advice of [David Robinson](https://twitter.com/drob/status/738786604731490304) for naming these chunks. It's a good idea to name chunks that create objects after the primary object that they create. That makes it easier to understand the `dependson` specification.
+    # chunk 1
+    `r chunk`{r raw_data, cache.extra = file.info("a_very_large_file.csv")}
+    rawdata <- readr::read_csv("a_very_large_file.csv")
+    `r chunk`
+
+As your caching strategies get progressively more complicated, it's good idea to regularly clear out all your caches with `knitr::clean_cache()`.
+
+Note that I've used the advice of [David Robinson](https://twitter.com/drob/status/738786604731490304) for naming these chunks. It's a good idea to name chunks that create objects after the primary object that they create because that makes it easier to understand the `dependson` specification.
 
 ### Global options