How to describe someone who's...uh...you know...Duh
• ~ Lights are on, nobody's home.
• ~ A few clowns short of a circus.
• ~ A few fries short of a Happy Meal.
• ~ A few peas short of a casserole.
• ~ Doesn't have all his cornflakes in one box.
• ~ Gates are down, the lights are flashing, but the train isn't coming.
• ~ The wheel's spinning, but the hamster's dead.
• ~ One Fruit Loop shy of a full bowl.
• ~ One taco short of a combination plate.
• ~ Fell out of the family tree
• ~ A few feathers short of a whole duck.
• ~ The cheese slid off his cracker.
• ~ Body by Fisher; brains by Mattel.
• ~ Couldn't pour water out of a boot with instructions on the heel.
• ~ An intellect rivaled only by garden tools.
• ~ As smart as bait.
• ~ Chimney's clogged.
• ~ Elevator doesn't go all the way to the top floor.
• ~ Forgot to pay his brain bill.
• ~ His sewing machine's out of thread.
• ~ If she had another brain, it would be lonely.
• ~ Slinky's kinked.
• ~ Surfing in Nebraska.
• ~ Too much yardage between the goal posts.
• ~ Big like ox; smart like tractor.
• ~ A few sandwiches short of a picnic.
• ~ Not wound too tight.
• ~ Shirt size: large. Cap size: small.
• ~ Room temperature IQ.
• ~ A few box cars short of a full trainload.
• ~ Missing a hard-drive.
• ~ Not the sharpest knife in the drawer.
• ~ Doesn't have two wits to bat together.
• ~ Not the brightest crayon in the box.
• ~ One tire short of a Goodyear.
• ~ A few Pinata's short of a fiesta.
• ~ Butter slipped off the noodles.
• ~ Not firing on all thrusters.
• ~ Two cards short of a full deck.
• ~ Three dots short of an ink-blot test.
• ~ Forgot to use a surge protector.
• ~ Half bubble off level.
• ~ The mental agility of a used soap dish.
• ~ Not the sharpest tool in the shed.
• ~ Rowing around the lake with one oar.
• ~ Couldn't pour water from a boot even if the instructions were written on the heel.
• ~ A few hairs short of a wig.
• ~ Not the sharpest pencil in the box.
• ~ Not the brightest bulb on the tree.
• ~ One flower short of a bouquet.
• ~ One can short of a case.
• ~ One Easter Egg short of a basket.
• ~ When they were handing out brains, he must have heard "trains", and well, he ordered a boxcar.
• ~ Not the greenest cucumber in the field.
• ~ Three feathers short of being fluffy.
• ~ The dip stick doesn't reach the oil.
• ~ Driveway doesn't go all the way to the road.
• ~ A few bricks shy of a load.
• ~ Smart as a box of rocks.
• ~ Bright as a burnt out light bulb.
• ~ Sharp as a bubble.
• ~ Nice toy--no batteries.
• ~ He shook the family tree and all the fruits and nuts hit his head.
• ~ A few skeeters short of a swarm.
• ~ The engine's on, but it's not in gear
• ~ A few fish short of a limit.
• ~ Not just up the creek, the water's all dried up.
• ~ Not firing on all cylinders.
• ~ The horse is running but the rider's dead.
• ~ A few pickles short of a full barrel.

And from a DownUnder friend:

~ Kangaroos loose in the top paddock (hopping around all over the place)
~ A sausage short of a barbie (barbeque or bbq?)