A/N: Changing things up a bit this time, and putting the prompt at the end. You'll understand why when we get there. ;)
Watson
"Tell me about one of your cases," said my dear wife Mary, late one winter's evening.
"Of course," I acquiesced with a smile. After all, I usually enjoyed telling them as much as she did listening, and so I spent the next minute or two in a thoughtful silence.
"Now I remember a good one," said I. "Never before had the solution to a case rested upon such a strange and innocuous detail than it did in the case of one particular kidnapping."
"John," said my wife, with little grin, "I believe you're telling your story backwards again."
"Allow me to begin at the beginning, then," I returned, returning her smile. "A distraught young man—I can't remember his name at present, I'm afraid—but he came to Holmes, telling him that he had received an unsigned, unaddressed letter demanding a large sum of money for the return of his four-year-old son.
" 'I believe this is a matter for the police,' Holmes told the man, in a gentle but firm tone.
" 'I have gone to the police,' said the young man. 'I spoke with Inspector Lestrade this morning, but I will not feel easy in my mind unless you are on the case.'
" 'I shall at least take a look at the letter,' Holmes acquiesced, 'and then I shall decide whether it would be best to leave the matter in Lestrade' hands, or take it upon myself.'
"The young man shifted uncomfortably. 'Inspector Lestrade is keeping it as evidence.'
" 'Then I suppose we shall have to call on Lestrade,' said Holmes, obviously a little irked by this development.
"The three of us drove to Scotland Yard. Holmes asked the young man a number of questions on the way, I believe about his family and servants. Our client lived with his wife and son, and several servants, most of them trustworthy. His brother lived a short distance away. All other relatives were much farther.
"We soon arrived at Scotland Yard, and met with our friend Lestrade. He was a little reluctant to give us access to the letter, but seeing the young man's distress, soon handed the letter to Holmes.
"My friend quickly skimmed the letter, and set it aside. He then picked up the envelope, examined it closely, and turned to your client, saying, 'I expect we shall be able to identify the kidnapper and find your son by this evening. Good day, gentlemen!'
"Both Lestrade and the young man and looked a little startled, and I must say that I was as well. Neither of us could see what was so singular about the envelope.
"I ventured to ask Holmes about it during our cab ride back to Baker Street, but he only gave a little chuckle. 'Just a little theory, Watson, that is all.' "
I paused in my narrative, seeing that Mary was beginning to show signs of drowsiness.
"Would you like to go to bed, dear?" I asked.
She shook her head, with a sleepy smile. "I must know how it ends first."
Then I would have to keep the rest as short as I could.
"Well, Holmes quickly amassed a short list of all suspects, and we drove to each of their living quarters in turn. At each location, he would ask to see where they would write letters, and make a close examination of the place, opening drawers, and staring very closely at the surface of the tables, his aquiline nose barely an inch away from the surface."
"You shouldn't be so mean about his nose," Mary chided me.
I shook my head. "Perhaps not, but it does look rather birdlike when he does that sort of thing. May I continue?"
Mary gave a quiet giggle. "Of course, John."
"Anyway, Holmes looked very closely at the place where his suspects would most likely write a letter, but nowhere else. At our fifth stop, which was the home of our client's brother. Holmes once again drew close to the writing desk, and began to examine it. Then I saw him stiffen, glance into a couple of drawers, and then rise again to his full height.
" 'I have found our kidnapper, Watson," said he in a low voice, an excited glint in his eye. 'We must inform the good Inspector, and have him arrested.'
" 'And the child?' I asked.
" 'Is surely secreted somewhere in the house,' he replied.
"Holmes was correct in every particular. The brother had kidnapped his nephew for a ransom, as his gambling habits had driven him to the point of near bankruptcy. The child was found and returned to his father, and the kidnapper was brought to justice."
"But how did Holmes do it?" asked Mary, eyes wide with interest.
I grinned widely. "Don't you have some theory?"
Mary's brow furrowed in thought. "Well, something about the envelope was connected to the place where they wrote it. He must have seen something in common between them. Was it the kind of envelope used?"
"No," I replied.
Mary thought another long moment. "Could you provide me with a hint?"
"Well, it wasn't something he saw at all."
Mary threw her hands in the air in mock despair, though her quiet smile told me she was only teasing. "Just tell me, John, I am never going to guess it."
"Holmes noticed the smell of peppermint. The man had just eaten a humbug when he licked the envelope to seal it, and Holmes smelt the candy at the man's desk, and found a small assortment of humbugs stored in a drawer."
Mary gave a laugh. "What a strange thing for a whole case to hinge upon!"
"I must agree," I replied. "Anyway, it's made me take more notice of the peppermint smell of humbugs since then!"
December 13: "Peppermint." (from Wordweilder)
A/N: Thanks for bearing with me on holding back the prompt. After all, I couldn't spoil the whole story before it began!
Also, I realized after I'd written most of this story that licking envelopes might be anachronistic, but I can't seem to find any information online about how long that icky-tasting glue stuff has been around, so I don't really know. I hope you enjoyed the story regardless. :)