Lucy remarks about Charlie Brown's annoyance with her asking him to do something. This is another case of a character's personality becoming defined from another character's verbal recognition of it.
That happens because comics use exaggerated behavior as a way to communicating effectively to the reader. To show anger, you show a character actually kicking the thing he's angry at, even though a real person would not usually do such a thing. It illustrates anger effectively however, and I think readers subconsciously recognize this and adjust their expectations. But it also means that, to actually establish a character's personality, you have to describe it explicitly somewhere, and in a strip that doesn't (generally) use narration like Peanuts you have to do that by putting that description in the mouth of another character.
Schulz would become quite masterful about adjusting reader expectations. His characters are able to act out theatrically when necessary, but can also play it very far down at times.
I also like the serif lettering on "RATS!" in panel 7.
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