secret jokes

Sometimes Mike and I are sitting near each other, both minding our own business on our laptops, and Mike starts busting up laughing, and I look over at his screen to see nothing but a straight logic puzzle.

This happens hilariously often.

He insists that he's just laughing at amusing things that cross his mind while he's solving, but I can't help wondering if there's some punch line I'm missing in these Slitherlinks.