What happens when you remove Garfield's thought bubbles, and in some cases explain the comic strip like Joe Mathlete Explains Today's Marmaduke?
Explanation 1: Jon was in a hallucination in the first two panels, and has just come out of it in the third.
Explanation 2: Jon has gone mute.
In both scenarios, Garfield is eating Jon's cereal and taunting him with it.