Boo!

Charlie and I are sitting in the green room, reading a story. We hear John coming up the stairs.
Charlie: "Daddy! Daddy! I hide Daddy!" (dives under bedsheet, giggling)
John: (entering room) "Where's Charlie? Oh, where did Charlie go?"
Charlie: (jumping out dramatically) "BOO! BOO!" (turning to me) "I boo Daddy!"
John: "Boo? I've never heard you say 'boo' before. Where did that come from?"
Charlie: (pauses thoughtfully, then points to his face) "From my mouth, Daddy!"