diff --git a/card.py b/card.py
index 7caef61..95f8f04 100644
--- a/card.py
+++ b/card.py
@@ -27,6 +27,7 @@ class Card(turtle.Turtle):
 
     def is_mouse_over(self, x, y):
         # Collision code reused from D. Atkinson's Turtle Crossing program, with some minor modifications.
+        # https://www.youtube.com/watch?v=dQw4w9WgXcQ
         top_edge = self.ycor() + 103
         bottom_edge = self.ycor() - 103
         car_left_edge = self.xcor() - 103