Idiotic Idioms

I've definitely had times of "being slow on the uptake."

There are many phrases that can express my mental slowness:
"He's not the sharpest knife in the drawer."
"The lights are on but no one's home."

Create a new phrase (or send in another classic one.)

Please indicate whether your phrase is a new one or one you've heard.

Examples:
(new, I think) "Let's just say, he's easily distracted by shiny objects."
(old) "The elevator doesn't go all the way to the top."