diff --git a/lists.Rmd b/lists.Rmd
index 27f7d04..258f594 100644
--- a/lists.Rmd
+++ b/lists.Rmd
@@ -192,6 +192,7 @@ results
 If you do this a lot, you should probably make a function for it:
 
 ```{r}
+df <- data.frame(x = 1:10, y = rnorm(100))
 col_medians <- function(df) {
   out <- vector("numeric", ncol(df))
   for (i in 1:ncol(df)) {