I heard the following dialouge from a moive:
Boy's mum: What do we do with the eggs?
Boy's dad: Give them back.
Boy: Well, what do I say?
Boy's dad: Tell her we don't eat eggs. We're allergic to them or something. Come on, use your brains.
I suppose the boy has only one brain, why does his dad say "brains".
It's one of the conventions in English that we refer to the actual organ in our head as our "brain", but we call our intelligence/reasoning ability/initiative our "brains". In this case, the father is telling the boy to use his reasoning ability, and hence his "brains".
I'm not a teacher of English, but I have spoken it for (almost) all of my life....