A/N: K rated post TFP family squabbling from John's POV.
"There's no right side to this."
"Yes there is. And it's my side, in case you were wondering. Yours is absolutely, positively WRONG."
Sherlock and Molly stood nose to nose, breathing heavily, glaring into one another's eyes. His neck was bent forward, hers tilted backwards, and both held their arms folded tightly across their chests.
"What's this about? Trouble in paradise?" John wondered as he took in the scene before him. Rosie babbled her own opinion of her godparents' combative stances as John set her on the floor and allowed her to toddle over to them.
Sherlock was the first to break the staring match as Rosie tugged at his trousers. "Up! Up!" she demanded, and he lifted her into his arms.
"I still say there's no right side," he said, somewhat sulkily, as Rosie immediately toppled herself into Molly's waiting arms.
"And I still say you're wrong," Molly shot back before pressing a kiss to Rosie's soft, pink cheek. "How's my Rosie-girl doing today? Did you have fun with Daddy at the park?"
John looked from Sherlock to Molly and back again. "Right. What's going on?"
Sherlock, pouting like Rosie denied - well, anything these days, the terrible twos weren't misnamed in the least - flopped onto the sofa, arms still folded across his chest. "Molly won't admit that hot chocolate from a packet tastes the same whether you make it with boiled milk or water."
"Wrong!" Molly sang out. "Tell him he's wrong, John. I mean, would you serve your daughter such an awful thing?"
"She's a bit young for hot chocolate at the moment, but sorry, Molly, I have to agree with Sherlock on this one." John shrugged apologetically. "It tastes the same to me no matter how you make it. Too sweet and artificial either way." He made a face, which caused Rosie to laugh and clap her hands.
Sherlock gave Molly a triumphant look; she stuck her tongue out at him, which made Rosie giggle even harder.
John simply shook his head. He never could understand how his two friends could resolve any serious issues that arose in their relationship almost immediately - but would allow the most trivial, ridiculous things to set them sniping at each other for days at a time.
Thankfully, this wasn't one of those times. "Let's do an experiment," Molly suggested, snuggling next to Sherlock and setting Rosie onto his lap. "Different brands of powdered chocolate, different kinds of water and milk...what do you think?"
Sherlock grinned at her. "Perfect!" he exclaimed, leaning forward to give her a quick peck on the lips. "A blind taste test it is!"
John smiled benevolently at the pair as he settled into his old chair in front of the fireplace, enjoying the sight of his friends' restored domestic harmony. If only The Great Biscuit Debate had been this easy to resolve!