We say "Dumb as a BAG of hammers."
My brother describes this kid to me this a.m. He says, "The kid is as dumb as a box of rocks, not even funny."
What did the kid do? He was trying to knock a truck tire and rim off of the hub with a 10 lb sledge. After a few attempts he gets real mad at it and takes his best swing. Hits the tire instead of the rim. 10 lb sledge bounces back and catches him SQUARE IN THE NOSE. Shattered it and the surrounding bone structure. He'll be going infor a little surgery once the swelling goes down.