diff --git a/ch1/ch1-04.md b/ch1/ch1-04.md
index f47b650..b7d53da 100644
--- a/ch1/ch1-04.md
+++ b/ch1/ch1-04.md
@@ -33,6 +33,10 @@ const (
 )
 
 func main() {
+	// The sequence of images is deterministic unless we seed
+	// the pseudo-random number generator using the current time.
+	// Thanks to Randall McPherson for pointing out the omission.
+	rand.Seed(time.Now().UTC().UnixNano())
 	lissajous(os.Stdout)
 }