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Black sheep dwell in every fold; All that glitters is not gold; Storks turn out to be but logs; Bulls are but inflated frogs.
— W.S. Gilbert • Things are seldom what they seem (song) from H.M.S. Pinafore

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
These verses list familiar cases of deceptive appearances: flocks hide black sheep, glittering things may not be gold, storks may prove to be logs, and bulls mere puffed-up frogs. The playful catalogue insists that surface impressions cannot be trusted.
Context reviewed