John hums happily to himself as he runs his fingers through Sherlock's hair, eliciting the occasional contented purr.
"I'm jealous, you know. It's not fair that you get to be tall, smart, gorgeous, and have this mop."
As his fingers continue to card out the dark curls, he can feel Sherlock's body tense slightly. He pauses.
"Something wrong?"
"I never realised you were that fond of my hair."
"Well, it is pretty spectacular."
"Yes, but..."
Sherlock sits up, his dark mane now in a ridiculous state - flat where he'd been lying on John and sticking out in all directions on the side he'd been playing with. John quirks an eyebrow, encouraging Sherlock to continue.
"I'm sure you've noticed, Mycroft's hair is thinning. Our father's hair also thinned early, as did both grandfathers. It stands to reason that eventually I will follow suit..." he trails off, biting his lip nervously.
John does his best to bite back a smile, Sherlock's obviously quite serious and nervous about this. Eventually though, a giggle slips out. Sherlock pulls away from him.
"I should hope you're not laughing at me."
"You vain, adorable man. You should know by now that I'm in this for the long haul. I assure you, I will still love you, impossible though you may be, even if you go bald."