My friends and I were talking about a girl we knew who wasn't exactly the sharpest tool in the shed...when one of my friends commented, "She's a few fries short of a happy meal."
Since then we've been researching on and thinking of witty comebacks regarding dumbness (the simple joys of life. haha.

) and I found this list :
Funny ways to call someone stupid: Sayings to describe people who aren’t smart or bright
Some of my favorites from the site

:
He’s all foam no beer.
His wheel is spinning, but the hamster is dead.
Some drink from the fountain of knowledge, but he just gargled.
Any you'd like to add to the list?