It was late afternoon of a chill November in Paris. I was walking along the quai, lost in a brown study. Looking up, I saw my friend C. Auguste Dupin approaching me.
"Ah, bon soir, mon ami," said Dupin, "and what brings you beside the Seine on this crisp evening?"
"Something has been puzzling me," I replied, "and I thought the walk would give my thoughts an opportunity to put themselves in order."
"It is a day of puzzlements, no? Will you not share your puzzlement with me?"
"Well," I demurred, "it is a small thing -- probably something I did to m…