diff --git a/transform.Rmd b/transform.Rmd
index aad611f..6eb3513 100644
--- a/transform.Rmd
+++ b/transform.Rmd
@@ -832,7 +832,8 @@ daily %>%
     `flights %>% group_by(carrier, dest) %>% summarise(n())`)
 
 1.  For each plane, count the number of flights before the first delay 
-    of greater than 1 hour.
+    of greater than 1 hour. (Hint: read about cummulative window functions and
+    see how their negations work, `vignette("window-functions")`)
 
 1.  What does the `sort` argument to `count()` do. When might you use it?