--- a/mamba/snake.py	Sat Sep 17 01:52:54 2011 +0200
+++ b/mamba/snake.py	Sat Sep 17 01:53:44 2011 +0200
@@ -2,6 +2,7 @@
 
 import random
 
+import pygame
 from pygame.sprite import Group, spritecollide
 
 from mamba.constants import TILE_SIZE, UP, DOWN, LEFT, RIGHT
@@ -92,6 +93,7 @@
                 self.shiftup_segments(1, tile_state)
 
         self.check_self_crash()
+        self.check_on_screen(world)
         for segment in self.segments:
             segment.shift_pixels(ds)
             world.interact(segment)
@@ -124,6 +126,13 @@
         if [s for s in collides if s not in self.segments[:2]]:
             self.crash('You hit yourself!')
 
+    def check_on_screen(self, world):
+        if self.coiled:
+            return
+        world_rect = pygame.Rect((0, 0), world.level.get_size())
+        if not world_rect.contains(self.head.rect):
+            self.crash('You left the arena!')
+
     def crash(self, reason='You hit something'):
         play_sound('crash')
         SnakeDiedEvent.post(reason)