Grace's best friend

Yesterday when Grace woke from her nap, I told her, "Guess who is coming over to play this afternoon?". No answer. Then I asked, "Who is your favorite playmate?". No answer. Finally I asked, "Who is your best friend?". Then Grace yelled out "Helen!" with a big smile. (Helen is her cousin, my sister's daughter.). I had no idea Grace even knew what a best friend was, much less that she already had one!