(as) dumb as a rock

Exceptionally foolish, dimwitted, or unintelligent. You thought you could fix the washing machine on your own, so you just started pulling it apart? I swear, sometimes you can be as dumb as a rock, Larry. I used to think that she was as dumb as a rock when we were kids, but she grew up to be a physicist. Are you dumb as a rock? Why would you touch the alarm when the sign clearly says "DO NOT TOUCH" in bold red letters?