diff --git a/documents/by-course/pstat-120a/course-notes/main.typ b/documents/by-course/pstat-120a/course-notes/main.typ
index f8b4f25..7469228 100644
--- a/documents/by-course/pstat-120a/course-notes/main.typ
+++ b/documents/by-course/pstat-120a/course-notes/main.typ
@@ -1323,7 +1323,8 @@ $
 for $k = 0,1,...,n$.
 
 The Poisson distribution is useful for finding the probabilities of rare events
-over a continuous interval of time.
+over a continuous interval of time. By knowing $lambda = n p$ for small $n$ and
+$p$, we can calculate many probabilities.
 
 #example[
   The number of typing errors in the page of a textbook.