If your house is like most with a toddler, you frequently hear "MINE!" Now that your house is full of toys from a holiday or birthday, you'll probably be hearing it even more.
According to Zero to Three, children don't have the capacity to learn to share until they're between 3.5 and 4 years old and don't fully understand the concept until they're even older.
This doesn't mean, though, that we can't continue to model sharing to our children. The sources below have a wealth of ways to encourage sharing.