diff --git a/visualize.Rmd b/visualize.Rmd
index 609c8d0..de09f56 100644
--- a/visualize.Rmd
+++ b/visualize.Rmd
@@ -163,15 +163,15 @@ ggplot(data = mpg) +
 
 Here, the color doesn't convey information about a variable, but only changes the appearance of the plot. To set an aesthetic manually, set the aesthetic by name as an argument of your geom function. You'll need to pick a value that makes sense for that aesthetic:
 
-* the name of a color as a character string.
-* the size of a point in mm.
-* the shape as a point as a number, as shown below.
+* The name of a color as a character string.
+* The size of a point in mm.
+* The shape of a point as a number, as shown below.
 
-R has a set of 24 built-in shapes, identified by numbers:
+R has a set of 25 built-in shapes, identified by numbers:
 
 ```{r echo = FALSE, out.width = "75%", fig.asp = 1/3}
 shapes <- tibble(
-  shape = c(0:19, 22, 21, 24, 23, 20),
+  shape = c(0, 1, 2, 5, 3, 4, 6:19, 22, 21, 24, 23, 20),
   x = (0:24 %/% 5) / 2,
   y = (-(0:24 %% 5)) / 4
 )
@@ -186,6 +186,8 @@ ggplot(shapes, aes(x, y)) +
   theme(aspect.ratio = 1/2.75)
 ```
 
+Note that there are some seeming duplicates: 0, 15, and 22 are all squares. The difference comes from the interaction of the `colour` and `fill` aesthetics. The hollow shapes (0--14) have a border determined by `colour`; the solid shapes (15--18) are filled with `colour`; the filled shapes (21--24) have a border of `colour` and are filled with `fill`.
+
 ### Exercises
 
 1.  What's gone wrong with this code? Why are the points not blue?