diff --git a/falling_sand.py b/falling_sand.py
index 647efad..d883e8d 100644
--- a/falling_sand.py
+++ b/falling_sand.py
@@ -115,7 +115,7 @@ class FallingSand:
         self.mouse = pygame.mouse
         self.mouse_pressed = self.mouse.get_pressed()
         self.mouse_pos = self.mouse.get_pos()
-        self.sand_surface = pygame.Surface(self.screen.get_size())
+        self.sand_surface = pygame.Surface(self.settings.window_size)
         self.sand_surface.fill(self.gray)
         self.matrix = pygame.PixelArray(self.sand_surface)
 
@@ -187,7 +187,15 @@ class FallingSand:
         print("Bye!")
 
     def window_update(self, size):
+        w, h = size
+
         self.settings.window_size = size
+        self.matrix.close()
+        old_sand_surface = self.sand_surface
+        self.sand_surface = pygame.Surface(size)
+        self.sand_surface.fill(self.gray)
+        self.sand_surface.blit(old_sand_surface, (0, 0))
+        self.matrix = pygame.PixelArray(self.sand_surface)
 
     def spawn_sand(self, position):
         for ax in range(-8, 9):