Colorful Metaphors For Stupidity:

 
 1.  Not the sharpest knife in the drawer.
 
 2.  A few clowns short of a circus.  

 4.  An experiment in Artificial Stupidity.
 
 5.  The wheel's spinning, but the hamster's dead.
 
 6.  All foam, no beer.
 
 7. Has an IQ of 2 but... it takes 3 to grunt.
 
 8. Warning: Objects in mirror are dumber than they appear.
 
 9. Couldn't pour water out of a boot with instructions on the  heel.
 
10. As smart as bait.

11. Chimney's clogged.
 
12. Sharp as a bag of wet mice.
 
13. Her sewing machine's out of thread.
 
14. His antenna doesn't pick up all the channels.
 
15. If he had another brain, it would be lonely.
 
16. Proof that evolution CAN go in reverse.
 
17. Receiver is off the hook.
 
18. Skylight leaks a little.
 
19. Too much yardage between the goal posts.
 
20. In the pinball game of life, his flippers were a little
    farther apart than most.