a Charlie funny

conversation between David and Charlie:

David: What does the cow say?
Charlie: mooo!
David: What does the sheep say?
Charlie: baaa!
David: What does the horse say?
Charlie: naaa! (close enough)
David: What does the pig say?
Charlie: (wagging a finger) no, no no!

Bonus points to anyone who knows why without reading the explanation below. ;-)

One of the kids' favorite board books is Sandra Boynton's Moo, Baa, La La La. The text of the book is:

A cow says, "Mooo."
A sheep says, "Baaa."
Three singing pigs say, "La la la!"
"No no," you say, "That isn't right! Pigs say oink all day and night!"
...and it continues

So, in Charlie's reading, the pig says no. Funny buddy.