He's about as sharp as a marble.
She's not the brightest bulb on the Christmas tree.
I wish I had a blueprint for his brain; I'm trying to build an idiot.
The cheese slipped off her cracker.
That guy is as smart as bait.
His elevator doesn't go all the way to the top floor.
She forgot to pay her brain bill.
His antenna doesn't pick up all the channels.
If she had another brain, it would be lonely.
He's missing a few buttons on his remote control.
Her receiver is off the hook.
The lights are on, but nobody's home.