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The raindrops ran down the windows of 221B Baker Street while Sherlock Holmes played a sad melody upon his violin. The weather and the music reflected his mood, for it had been a trying and vexatious twenty-four hours. It was rare for a problem to stymie him like this, but try as he might to distract himself from the perplexing riddle that had been posed to him, his mind always found a way to wander back to it.

With an angry cry, Sherlock cast down his violin and hunched over the crumpled paper once more.

“Something the matter, Holmes?” inquired Dr Watson from his chair by the fire. Sherlock rounded on his roommate in frustration and disbelief.

“Deduce for yourself, John. My crying out, my ill treatment of my instrument, my tormented posture – are these the actions of a man for whom nothing is the matter?!”

Sherlock turned back to the note on the table and muttered under his breath…

“No cipher that I have ever seen… The lower case letters simply must be important – else why would they be there?... A substitution perhaps, but then also…”

Dr Watson ventured to speak again. “You are still considering the importance of that note then?”

“Considering?!” fumed Sherlock. “Watson, I am far past merely ‘considering’. I am consumed. My thoughts, my words, my very being – all of it is drawn towards this exasperating code… and yet I am unable to decipher it at all! What meaning can it possibly convey?! It bears all the hallmarks of Moriarty’s twisted mind – he must not be permitted to defeat me. See!”

Sherlock thrust the paper out towards his colleague and friend; the letters of its message stood black in the fire light.

SH,

YOU CaN SeArCH ThIS LaNd. YOU CaN UPtURn MoUNTaINS.
YOU CaN PrOBe, SnOOP, PrY As OFTeN, As SWIFTlY, As CoNSTaNTlY As YOU WISH.
I ScOFF At YOU. I SnICKEr. I BrAg.
ONCe SUN HAs ArISeN – IF YOU Be NO WISEr – I WIn.

Dr Watson stroked his chin and opened his mouth. “Suppose that this-” But before he could go further Sherlock had torn the page sharply away from him.

“Suppose nothing, Watson. If this problem troubles me – the great Sherlock Holmes – to this extent, what measly hope is there of your having a solution? Take yourself off to bed – you shall be of far more use elsewhere. Meanwhile, I shall not have rest until the answer is in my hand and Moriarty’s enigma lies shattered, whispering all of its secrets to me.”

“If you insist, Sherlock,” replied Watson, removing himself from the room, a knowing look upon his face.

Sherlock laboured all night. With great howls and shrieks, he toiled and slogged, but when the dawn came and Dr Watson re-entered the room after a good night’s sleep, he found his associate prostrate on the floor, bedraggled and groaning as if in great agony.

“I am vanquished, Watson. As Queen Victoria lives and breathes, I have been unable to fathom the answer. I must concede. Moriarty is victorious.”

“No,” said Dr Watson. Sherlock looked up.

“No?” replied Sherlock through bloodshot eyes. “How can Moriarty not have won? I have failed to solve his cipher.”

Dr Watson smiled. “I say ‘No’, because it is not Moriarty’s cipher.” Sherlock frowned. “It is mine, Sherlock. In fact, it is not even strictly a cipher – it is merely a trifle, of my own devising. An attempt to demonstrate to my close friend – to you – that sometimes I do have an intellect, as overshadowed as it may be by your own. And as you can see, the sun has arisen – and you are no wiser. Therefore, I win!”

Sherlock stood up and laid a hand on Dr Watson’s shoulder. “Then I congratulate you, Watson. It is you, and not Moriarty, who has bettered me on this occasion. I stand here a humbler and less conceited man. But pray, tell me, what is the secret to your riddle?”

Dr Watson grinned. “It is very simple, Holmes – it is __________.”

Sherlock closed his eyes, abject. “Of course, of course.” Then he looked askew at his conqueror. “But in that case Watson, you have made one mistake…”

What one word did Dr Watson say to Sherlock to explain his riddle? And what is the mistake which Sherlock has spotted? Both answers are required for a complete solution.

Answer:

It is very simple, Holmes – it is elementary.

Explanation:

The entire message is composed of the abbrevations for chemical elements. The lowercase letters appear whenever there is a two letter symbol. For example, the word "CaN" includes the symbol "Ca" for Calcium followed by "N" for Nitrogen. The word "SeArCH" is composed of "Se" (Selenium), "Ar" (Argon), "C" (Carbon), and "H" (Hydrogen). .

The mistake:

In the word "At", the element At (Astatine) was only discovered in 1940, which means Watson and Sherlock would not have known this symbol.

Partial answer: the mistake is that

the end of UPtURn was unknown thus until 1923.