"Curve your pinky, Hamish," Sherlock instructed. The young boy stuck out his tongue in concentration, tiny fingers clutching the wooden bow.

"Yes, like that. Now put the bow on the E string."

"Which one is the E string, Papa?" Hamish asked.

"The top one. Now put your bow on it, that's it, now pull the bow down, nice and straight."

Scrrrriiiiiiiichhh.

"Not so much pressure, Hamish. Gently," Sherlock corrected.

Screee.

"Closer." Sherlock picks up his own instrument. "Watch me."

Sherlock draws out a long, pure, ringing note on the small instrument.

"Now, try again."

Eeeeee, Hamish's tiny violin cries out.

"That was very good! Well, the best yet, at least."

They are interrupted as John opens the door to the flat with three bags full of groceries.

"Ah, music lesson time?"

Hamish gently sets his violin down on the sofa and then rushes over to John, hugging him around his knees. The young boy looks up, still clutching tight, and says, "Papa won't teach me any songs, Daddy. Make him teach me a song!"

Chuckling, John exchanges a look with his husband, and turns back to their son. "You have to learn the notes first, Hal. Papa will teach you a song as soon as you know the notes, isn't that right?"

John shoots Sherlock a look that says you better say 'yes'.

"Of course. Now, are you ready to learn the next note?"

"Then do I get to learn a song?"

"Almost. Now, pick up your violin and hold it like I showed you."

John has never seen Sherlock so patient and gentle with anyone, not even John. He puts away the shopping and flips the kettle on.

Digging the tea tin out of the cupboard, John listens to the confidant scratches that his son makes.

Sherlock directs Hamish's fingers on the slender black fingerboard. "A, A, E—Switch strings, yes that's it—E, F—first finger, Hamish—E. Now all three right on the tape, tell me which note that is?"

"A, B, C," Hamish mutters under his breath counting to himself, "D! It's a D, Papa!"

"Quite right. Now, do you know what song you're playing? Which note comes next?"

"It's Twinkle, Twinkle, Little Star. But how do I know what note to play now?"

"You know the tune, figure it out. Sing it for me."

Hamish goes through the rhyme, and when he gets to the pitch where he's stopped, Sherlock points to the violin and says, "Now, match the pitch. Keep singing, and find the note."

He is gentle, careful not to rush the five year old. John removes the tea bags and brings mugs over for Sherlock, Hamish, and himself.

Hamish loves to drink tea just like his Dad, and not even the lure of finishing a song can stop him from mimicking John's every sip.

If he looks back on it, John would never have imagined finding such utter, domestic bliss in a marriage to a genius consulting detective with a son who isn't (genetically) theirs.

But it fits, John decides as Sherlock sighs at the caffeinated disruption, and he wouldn't change it for the world.