...and then you tell them that it's exactly like someone writing a fake return address on a piece of mail before sending it to you, and they understand, because

...and then you tell them that it's exactly like someone writing a fake return address on a piece of mail before sending it to you, and they understand, because as stated above, this is conceptually very simple. You don't need to talk about "headers" at all - I don't know why you're bringing them up.