diff --git a/lists.Rmd b/lists.Rmd
index bc39bb0..bb5105d 100644
--- a/lists.Rmd
+++ b/lists.Rmd
@@ -157,6 +157,7 @@ embed_jpg("images/pepper-3.jpg", 300)
 1.  Generate the lists corresponding to these nested set diagrams.
 
 1.  What happens if you subset a data frame as if you're subsetting a list?
+    What are the key differences between a list and a data frame?
 
 ## A common pattern of for loops