"The wheel is spinning, but the hamster's dead."
"A few tacos short of a combination plate."
"The elevator doesn't go to the top floor."
"The formula 'Two and two make five' is not without its attractions." --Dostoevsky
"A few bricks short of a load."
"A few french fries short of a happy meal.."
"Sharp as the edge of town."
"If she was any dumber, we'd have to water her."
"Doesn't have both oars in the water."
"Crazier than a skunk in a bucket full of hash."
" Not the brightest porchlight on the block by any means!"
"Not the sharpest tool in the shed."
"A few screws short of a hardware store."
"A burger short of a combo meal."
"The gates are down and the lights are flashing, but the train isn't coming."
"Has an IQ of room temperature."
"Couldn't pour water out of a boot with instructions on the heel."