Adding- You might guess by now that I do not force my kids to share. My DD's can't follow each other around saying "mine" but if they are actively using blocks they are not required to share with their sibling unless they want to. I encourage it by praising when they do choose to like saying "wow, sister looks so happy that you chose to share your blocks with her!" It is a choice though, just like I have a choice if someone asks to borrow my car.