Hi!

Melina is almost 4.5 years old.  She has been speaking clearly for a very long time, and has known and used the word Hi, since before she was 1 year old.

Melina:  Hi!
Me: Hi to you, too!
Melina:  Mom, what does Hi mean?
Me:  Hi?  You know what that means.
Melina:  Well, I don't really think that I do.
Me:  Well, it is a greeting.  It is something you say when you see someone.  You know?  As in, Hi, how are you doing?
Melina:  Well, no, I really don't think I know what it means.
Me:  ...Um...Okay.

I stood in the kitchen and looked at her.  Just looked at her.  I had nothing else to say.  I think sometimes she is teasing me.