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Explanation
The Joke
Two children discover a mermaid (or merman) washed up on the shore and are amazed. In subsequent panels, they are shown transporting the mermaid in a wagon, then walking toward a building labeled "City Pool." One child asks, "Do you really think this'll work?" and the other replies, "No, but if it does, we'll be gods." The final panel reveals their plan: they have put the mermaid into the public swimming pool, which has an "Adult Swim" sign -- the implication being that a mermaid in the pool during adult swim would be a spectacular and bizarre event.
The Humor
The comedy comes from the absurd gap between the magnitude of the discovery (a real, living mermaid -- one of the most fantastical beings imaginable) and the utterly mundane use the children put it to: sneaking it into the local city pool. Rather than contacting scientists, the media, or the government, these kids have the priorities of actual children -- they want to cause chaos at the pool. The line "we'll be gods" elevates their petty pool prank to mythic proportions, which is funny precisely because it is just a pool. The "Adult Swim" sign in the final panel adds another layer: the mermaid is the ultimate "adult swimmer," being literally half fish, and the children may be hoping that having a mermaid counts as having an adult present.