But I want it!

Why is it that children believe that Mommys have magic wands that they can wave and create whatever it is that said child wants at that very moment? For example:

Lucy: "Mommy, I want some juice!"

Me: "We don't have any juice. Do you want some milk?"

Lucy: "No, I want juice."

Me: "But we don't have any juice."

Lucy: "But I want some juice!"

Me: "Lucy, we don' t have any juice. I can't give you something that we don't have."

Lucy:"BUT I WANT IT!!" (go on keep yelling... because that makes me that more excited about helping you)

this exchange could go on and on, and it often does, which leads me to wonder, does Lucy believe I'm really a mean mommy in disguise that hides the juice for my own pleasures and maybe, just maybe, if she keeps demanding, I will produce the juice from some hidden corner of the kitchen? Or maybe she thinks I have a mary poppins bag style pantry and can just reach in and pull out, at any moment, what ever she desires. The exact same conversation could be applied to any number of situations... I want more popsicles/you can't have more popsicles; I want my purple socks/your purple socks are dirty. I'm not sure what to do about this dilemma, but it's driving me crazy!