A classic "Lucyfer" moment:
Linus: I hate to mention it, but you've been awfully crabby since
Christmas.
Lucy (scowling): Anybody who is at all
sensitive is bound to have a post-Christmas letdown!
Linus (raising his eyebrows in a hopefully-shrewd
expression): Isn't a post-Christmas letdown and being crabby really the same thing?
Lucy (outburstingly squalling at the top of her lungs, so that her mouth appears as a huge round black cavern): NOT AT ALL!!!
Linus (does his familiar "flip-flip" somersaulting over backwards in startled shock)