diff --git a/prototype/balls.html b/prototype/balls.html
index 93ad38f..2d34db8 100644
--- a/prototype/balls.html
+++ b/prototype/balls.html
@@ -4,6 +4,7 @@
 <script src="../src/input.js"></script>
 <script src="../src/vector.js"></script>
 <script src="../src/priorityqueue.js"></script>
+<script src="../src/observable.js"></script>
 <script src="../src/collision.js"></script>
 <script src="../src/physics.js"></script>
 <script src="../src/RenderObject.js"></script>