Get it?

Earlier...

 Uma (reading from a book of jokes): Why did the penguin have a long neck?
Mommy: ....
Uma: Because it ate a giraffe!

time passes...

Kieran: Knock knock.
Mommy: Who's there?
Kieran: Penguin.
Mommy: Penguin who?
Kieran: It has a long neck, but it's really wearing a giraffe costume!