"Don't touch that! It's Daddy's; Go Play in YOUR room; That's your sister's, put it back; This is Mommy and Daddy's room and these are OUR things, not yours; etc. ad nauseum. No, not exactly those words, but you get the idea.
A child is born w/out a sense of greed. We strove to prevent saying those types of things and it was not until my oldest started preschool that she coined her own word "mys" for private ownership. We hadn't used "mine, my, yours, ours, etc." in our discourse with her, but send her to preschool with a bunch of good capitalists and the first thing she learns is "mine". Swell.
Don't buy it? Take a 1 year old and show him/her a ball or doll or toy. They look at it, they play with it. Then what happens? They hand it to you. Sure, after a time, they reach out for it - THEY WANT YOU TO BE GENEROUS LIKE THEY ARE. THEY WANT YOU TO SHARE ALSO.
We do not teach them to share, they teach us. We teach them greed. And for arguably good reason - our nation is founded upon it.