use your brain(s)

[Winwin2011]

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".

Thanks.

 

[Grumpy]

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".