Olivia's First Joke

Olivia was laying on the couch with Leah, and then hit her on the top of
the head.

Olivia: Knock Knock
Leah: Who's there?
Olivia: Boo hoo.
Leah:Boo hoo who?
Olivia: (slight pause)...Daddy!
Leah:( longer pause)...I don't get it...