Where did the saying "make sure you wash behind your ears" come from?

Question by Damski007: Where did the saying "make sure you wash behind your ears" come from?
And what point where they trying to make?.
I know that you can't see behind your ears, but you can't see the back of your neck either!. And I haven't heard any saying about washing your neck!.
I was told by someone that it originated in church, as the choirboys had to show good upstanding and that the boys that stood behind could see if they where clean or not.
How true that is I don't know, hence me

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