When you brain is as useful as a rock.
Man, after hitting that blunt I got mad rock brain.
When someone has large rocks in place of an actual brain
"Panglou i can't believe you're that stupid, you have rocks for brains"
When you think of something bad that happen to somebody and, it really happens to that person.
I through a brain rock at that guy, hoping he would fall, and... well he fell!