Kids do the darndest things

What is it about children that allows them to manipulate parents (and step-parents) so effortlessly? I used to think that I was immune to Ellie's big blue eyes, and to some extent I am, but there are times when all she has to do is look at me and say PUHLEEZE, and I just melt. And God help me if she turns on the waterworks and starts to cry. That is the ultimate weapon. So the question is, is that an innate ability that all children possess? I must admit that I was a pretty good little manipulator when I was growing up, but where did I get that ability? It seems to be something that all children are born with, so why do adults always fall for it? Perhaps it's just the natural order of things.