I though this would be a fun diversion, to see how people describe those that walk among us that are not quite... well, you know. Here are a few to start you off:
He's "a couple of fries short of a Happy Meal..."
She's "one can short of a 6-pack..."
He's "down one quart of oil..."
She's "not the brightest bulb in the chandelier..."
You get the idea...