Mitch4 sends this in: “I guess it’s his own shadow he’s seen to be catching in the last panel. But how could he have snagged it in the previous panel, when he was on that medium-distant hill and not casting a shadow at all, as far as the drawing suggests?”
Similar shadow physics:
Comics I Don't Understand 
If the sun is at the proper angle, someone on the hill and someone on the shore could have the same point as the top of their shadow.
And you’re right about him apparently NOT having a shadow in the first panels… But that would have given away the joke.
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