I love a good metaphor. How can you not love a language that so eloquently says one thing when it means another? And since my humor tends to be a little sarcastic…I have an arsenal of metaphors ready to use at a moments notice if I see someone acting…less than intelligent.
The wheel's spinning, but his hamster's dead.
He doesn't have all his dogs on one leash.
He would be out of his depth in a parking lot puddle.
She's got a photographic memory but with the lens cover glued on.
If brains were taxed, he'd get a rebate.
If he were any more stupid, he'd have to be watered twice a week.
When his IQ reaches 50, he should sell.
Her slinky is kinked.
He's as sharp as a bowling ball.
He's all hat, and no cattle.
Of course, these are only meant to be used in the spirit of good humor, not in a mean or nasty context at all…unless the subject is out of earshot.