Sharing is already starting to become an issue in our household.  Matthew doesn't want to share any of his toys with Meredith because they are "special" to him but it's okay for him to play with her toys because he is "showing her how they work", even if she is right in the middle of playing it.  One of Matthew's favorite toys, Legos, is especially troublesome because he likes to play them all the time and gets upset if she even comes near them (not to mention the safety aspect of her putting the small pieces in her mouth).  To try to remedy this, Meredith has her very own girly legos.  She loves them and will play with them while Matthew plays with his.  Of course Matthew still feels that he needs to "show her how to build stuff" and proclaims that she is sharing with him and that she told him to teach her...... *sigh*