Cliché: the lights are on but there's nobody home

Meaning: someone who isn’t very smart. Use examples.
Rewrite 1: the lights are running on a weak generator³
Rewrite 2: the door is open but it leads nowhere
Rewrite 3: the draperies are open but no light’s coming in through the window
Rewrite 4: the faucet is open but no water’s coming out

Comment: A parallel rewrite is difficult to do without sounding pretentious, but there are a number of other ways to say “this light’s working on low wattage.”

More reading about clichés
What I found when I googled “clichés”:
• Dan Schneider on Redeeming Clichés: “…a 10 line poem with at least 8½ seeming clichés…”