A few clowns short of a circus.
A few fries short of a Happy Meal.
A few beers short of a six-pack.
A few peas short of a casserole.
Doesn't have all his Cornflakes in one box.
The wheel's spinning, but the hamster's asleep.
Dumber than a box of hair.
A few feathers short of a whole duck.
All foam, no beer.
Body by Fisher Price, brains by Mattel.
Warning: Objects in mirror are dumber than they appear.
Couldn't pour water out of a boot with instructions on the heel.
An intellect rivaled only by garden tools.
As smart as bait.
Elevator doesn't go all the way to the top floor.
Forgot to pay his brain bill.
Her sewing machine's out of thread.
His antenna doesn't pick up all the channels.
His belt doesn't go through all the loops.
If he had another brain, it would be lonely.
Missing a few buttons on his remote control.
Proof that evolution CAN go in reverse.
Receiver is off the hook.
Skylight leaks a little.
Surfing in Nebraska.
Too much yardage between the goal posts.
In the Pinball Game of Life, his flippers are a little further apart than most.

:P

I prefer my joke.

@ Empty :
Enlighten us with your joke :)

See my thread! I swear you wont be able to stop laughing!!

A couple more
One stop short of East Ham
Not quite the full dozen

A sandwich short of a picnic
A blind mouse short of a nuresry rhime
'tuppence short of a threpenny bit
3 slices short of a loaf
not a full shilling

Am sure there are a few more as well :D

I've got one for Empty,
A box that is empty, no brains...