Conversations.

Though it isn't exactly grammatically correct, Harry's speech is at the point where you can actually converse back and forth:

Harry: Where were you?
Me: I was downstairs.
Harry: : I'm not see you.
Me: I was in the bathroom folding laundry.
Harry: I not see you.
Me: Well that's because you didn't look in the bathroom.
Harry: I not looked there...
Me: You're silly
Harry: No, You're silly mommy. Silly goose. (roll eyes)