Some more on panels and how to use them. In this Peanuts strip, the full length of the panel is also utilized, but not to emphasize a distance. Instead, it is used in its temporal capacity. As we read from left to right, we perceive events in the left part of a panel to transpire before those in its right part. Going from left to right, we perceive a natural temporal progression, and this is used by Schulz in this strip to build up a tension, or perhaps an expectation, that falls flat in the last – that is, rightmost – part of the panel.
And then, finally, we come to Snoopy, who's fallen asleep and is falling off the bench.
This is a perfectly adequate joke in itself, well told, but it gets better thanks to Schulz having developed Snoopy's character for 40 years or so in the strip. It is perfectly logical for the egocentric Snoopy, who tends to live in a world of his own much of the time, to have been entirely unaffected by the pathos and dedication of Charlie Brown and the others -- and, as long-time readers of the strip, we're of course expected to know that. So the joke is, again, enhanced by the artistic skills of the creator, but this time those skills aren't primarily drawing skills but characterization skills and building up an internally consistent "universe" his characters live and interact in.
Schulz could have had Lucy and Snoopy swap places for this joke, of course -- Lucy is no less self-centered than Snoopy, and has a long history of not taking baseball as seriously as Charlie Brown (though, in fairness, nobody -- except Peppermint Patty -- takes baseball as seriously as Charlie Brown). Snoopy is a more physical character than Lucy, though, and thus more suitable to the falling-off-the-bench-with-a-KLUNK! part than she is, and more instantly recognizable when all we see of him is his rear and legs.
Schulz was a master of his craft. I miss him.