What an odd pregnant pause that is in panel 2, as Jon waits for a response from Garfield. So today the gag is that Garfield sort of silently "pleading the Fifth", as it were, until he cannot avoid the question, and his silence insults Jon by implication. That's a standard enough joke, but the complicated part for this strip is that Garfield can't speak; silence is expected from a cat in reply to direct questions.
Now, careful readers who are not smart-asses about such things, understand that while Jon cannot "hear" the cat, he is generally able to read Garfield's responses from nonverbal cues. This means Garfield is not only hesitant to provide Jon an answer he can understand (rolling his eyes, walking away, etc.), but also refuses to "think" an insult, which the reader and Garfield would enjoy. The timing of this gag involves a second panel that looks empty, but is actually playing with the expectations of the characters and reader, and the established rules that govern the storytelling. This is all in service of a bigger and better dis, which is to entirely ignore a man, and stare right through him, implying that his expression of concern for your feelings means nothing to you.