A bag of hammers
If someone is as dumb as a bag of hammers, they're the dumbest thing you've ever seen.
My cousin said the moon is made of cheese. I said he’s as dumb as a bag of hammers.
My teacher said 1+1 is 10. I told her she was as dumb as a bag of hammers.
My dad said the sun is a lightbulb. I said he was as dumb as a bag of hammers.