This one goes out to my girlfriend (who thinks that the medical procedure I wrote in for a seizure is bullshit which it is because I googled it) thanks love for inspiring this little story here.
Also, if you want a story similar to this by Sir Arthur himself might I suggest How Watson Learned the Trick? ;)
John watched the man at the table in the corner of the little coffee shop, trying to make it look subtle so as not to be noticed. Twice he had to look away before the man made eye contact, but so far he was undetected.
Well undetected by his subject at least.
"Why are you staring at that man?" Sherlock asked, his forehead crinkling the way it did when he was confused by the antics of mundane normal people.
"I'm deducing." John gave a small smirk, taking a sip from his cup.
"Really?" Sherlock smiled at his boyfriend's use of that trademark word. He turned and looked the man over himself and then turned back to John. "Alright, tell me what you know."
"Well he's got a girlfriend." John started, wearing his triumphant grin.
"Explain." Sherlock replied simply, watching John work with curiosity.
"Well who else would make him wear that neat button-up shirt? And he ordered for two so he's waiting for someone. His date obviously."
"Obviously." Sherlock grinned and chuckled. "Go on."
"He's waiting for a call from someone important, he's keeping his phone on the table and he keeps looking at it." John nodded towards the phone. "And he's got a textbook in his bag so he's got to be a student."John finished his deductions with a proud smile.
"Brilliant, John. Absolutely." Sherlock clapped quietly and praised his boyfriend. "Except it's all wrong." John's face fell slightly and his lips formed into a pout.
"What do you mean all wrong?" He demanded to know. Sherlock gave that characteristic "smarter than you" look and began his speech.
"The textbook doesn't belong to him, it's several sizes too big for that bag and if he was used to carrying it he would get a better bag, he's carrying it for someone. Probably someone forgot it at his house. As for the cell phone he's not waiting for a call he's checking the time. Both of these factor into your first deduction: the girlfriend. You see he's waiting for his date however..."
Suddenly the bell on the door of the little shop rang and a handsome young man walked in. The subject of Sherlock and John's deductions grinned and hastened to meet him, kissing him soundly on the mouth.
"I rest my case." Sherlock leaned back in his chair and took a sip of tea while John watched, his mouth agape with shock.
"Alright, well I thought I had your methods down..." John grumbled.
"You'll get there, you made some decent points. Besides I wouldn't keep you around if you weren't clever." Sherlock winked.
"Well I do have one deduction I know to be correct." John replied slyly.
"Oh? What's that?" Sherlock grinned as though expecting another challenge, what he did get though was a kiss on the cheek.
"I deduce that you'll rather enjoy tonight's 'activities'. So long as you keep your mouth shut about why you 'keep me around'."