diff --git a/falling_sand.py b/falling_sand.py
index 8e047cb..014b3c2 100755
--- a/falling_sand.py
+++ b/falling_sand.py
@@ -213,7 +213,7 @@ class FallingSand:
                 bx += ax
                 by += ay
 
-                if not bx > self.sand_surface.get_width() - 2 and self.matrix[bx, by] == self.sand_surface.map_rgb(self.gray) and bx % 2 == 0 and by % 2 == 0:
+                if bx < self.sand_surface.get_width() - 2 and by < self.sand_surface.get_height() -2 and self.matrix[bx, by] == self.sand_surface.map_rgb(self.gray) and bx % 2 == 0 and by % 2 == 0:
                     self.falling_sand_particles.append(FallingSandParticle(self, (bx, by), color))
 
     def rainbow(self):