Memories
When Watson would later ask him about it, he would claim he deleted the memories from his brain to make room for more important information. However, Sherlock had very distinct memories. Four memories, to be accurate, that he could not delete no matter how much he desired.
The first memory was when he first realized John had brought a cat to their 221b Baker Street flat. He had fallen asleep in his black leather chair, violin in his lap and gun on the side table (naturally). As usual, he was awoken by the sound of John's morning shower. However, he opened his eyes to see a mass of grey fur and two green eyes staring at him.
"Sherlock?! Sherlock, stop shooting the walls!" called John as he turned the water off in the shower. Now he would have to stop Sherlock, not get shot himself, and apologize again to Mrs. Hudson for the noise and damage, he thought.
"John!" shouted Sherlock in reply, "A cat has found its way into our flat John!" Three more shots rang out, along with a chorus of hisses, the scratching of nails on the wooden floors and crashing of glass.
"Sherlock!" John called as he put on his bathrobe and hurried out into the kitchen. Sherlock was standing in the middle of the living room aiming the gun at a running mass of grey fur scampering terrified across the wooden floor. "Stop! It's Sarah's cat."
Sherlock looked up at Watson with confusion. "And you decided it was an ingenious idea to bring it to our flat?" he said arrogantly, asking it as a question with no intended response.
"As I told you last night, Sarah's new building doesn't allow animals, so I offered to keep it for her," he said, a slight question in the second half of his answer, as his self doubt about bringing the animal to the flat crept into his voice.
"You never told me about this."
"I told you last night as you were playing the violin after dinner."
"I never listen to you while I play the violin."
"You should I have never thought you were listening," John said, more to himself than Sherlock. With that John turned around to get back in the shower as Sherlock dropped the gun back on the side table. John stopped for a moment and looked back at Sherlock glaring at the cat as it now sat on the mantle between the skull and pile of unopened mail, innocently licking its front paw and flicking its tail back and forth. "Don't shoot it. It's a cat, its name is Shadow, and it likes you, god only knows why. And I don't think Sarah will enjoy learning my flat-mate shot her cat."
"AND NO EXPERIMENTS ON THE CAT!" shouted John as an afterthought, but knowing Sherlock, the ideas were already beginning.
The second memory also involved John's intervention between Sherlock and the cat. It was a Tuesday and Sherlock was running a set of experiments on live bacteria and acids. His experiment had involved feeding E-Coli bacteria treated with sulfuric acid to the cat in order to determine its affect as the sulfuric acid began to kill the bacteria. However, Watson arrived back from the store earlier than expected to find Sherlock stirring the cat's food. He stopped and looked at Sherlock in amazement "You're feeding the cat. Last week you forgot to feed it while I was gone for a day and now I leave for one hour and you want to help? Alright then."
Sherlock stopped stirring and returned to his microscope. "How was the market?"
"Good", Watson replied launching into the thing he bought and ways he how he had almost gotten the cashier's number. He started loading the food into the refrigerator when he realized the Petri dishes of live bacteria were gone. "Sherlock, where are the bacteria?"
Without a reply, Watson looked over the cat eating its dinner. "Sherlock, I said no experiments on the cat." He said as he pulled the dish out from underneath the cat's mouth.
"If I'm right, which I always am, it won't cause permanent internal damage it the digestive track."
"Stop risking lives to fulfill your childish experiments."
"It's a cat John, and it bothers me when I'm trying to think."
"It a living creature, deal with it"
"I have to deal with small minded fools everyday in society, I shouldn't have to deal with even stupider creature at home" said Sherlock, again glaring at the cat.
John Watson sighed and went back to loading the food into the refrigerator while he tried to figure out a way to dispose of the cat food filled with unknown amounts of deadly bacteria.
The third memory was one of no true importance, yet Sherlock was unable to erase it from his memory. It was a gloomy Friday evening as Watson was out romancing yet another woman. Sherlock was sitting, still talking to him, absent mindedly plucking his violin. Suddenly, his train of thought was interrupted by the brush of fur against his arm and a small purr as the cat rubbed his head into Sherlock's shoulder. Sherlock sighed and looked at the cat's innocent closed eyes as it rubbed the crest of its head into his shoulder. He could see the hairs being left on his coat and the chair, but he just sighted and turned back to his violin. For some reason the cat kept coming back to him, despite his many attempts to kill it over the past few months it had lived in their flat.
"You really are a stupid cat" he said as it continues to brush against his shoulder.
The fourth and final memory was the most vivid, and actually did not involve the cat directly at all. Sherlock and Watson had arrived back at the door of the Baker Street flat late one evening after chasing a lead on their newest case all day. They unlocked the door of their flat and Sherlock noticed the shoe treads on the stairs, less than three hours old than matched neither his own, Watson's or Mrs. Hudson's shoes. They were a size 10 man's shoe. Moriatry's size.
"He's been here" he said as he quickly began to climb the stairs with caution, thinking of the possible reasons for Moriarty's break in, the possible hostages, possible traps, and the millions of other calculations running through his head. Watson silently followed Sherlock, not even having to ask who the "he" Sherlock was referring to was.
As the two men crept up the stairs and the door came into view, Sherlock knew exactly what had happened.
The collar of the cat was hanging on the door, with a nail in each end of the stiff leather collar. One of the nails also held a note typed on a classic Ultra 2000 Typewriter 1946 editions, reading; "Good Kitty". The amount of pressure needed to nail it there, brand of nail, type of hammer, type of metal in the nail and millions of other variables ran through Sherlock's head as he slowly opened the door. Watson stood a few stairs behind Sherlock, and Sherlock brushed past him as he turned and descended the stairs as soon as he knew Moriarty was gone from the room. He had left his message inside the apartment, and Sherlock knew he would be long gone by now. "Dispose of it before it starts to smell, I can't think with that in the flat," Sherlock said knowing Watson would only hear the impartiality in his voice.
Sherlock didn't listen for Watson's shock, the emotional reaction, or the calling for Sherlock to return. He had to get out of the flat before he could start to really look at the scene, analyze the details. He had to get out before a new memory could fully form. He had to get out before the memory of Watson removing the corpse could exist.
Because that was something that would bother an ordinary mind.