"Sherlock?"
"Hm?" was his distracted reply, not even looking up from whatever he was doing at the moment.
"Why aren't you blonde?"
Sherlock slowly turned towards John with a mixture of surprise, confusion, and faint incredulity on his face. "What?" he mumbled, still slightly off in his own world.
"Why aren't you blonde?"
At the repeat of this admittedly extremely strange question, Sherlock seemed to come to his senses. "Why on earth do you ask, John?"
"Your name." John replied simply.
"Yes, what about it?" prompted Sherlock, his voice clipped and impatient.
John's eyebrows shot up in surprise. "Why, surely you, of all people know!"
"What is it John, what is it? I'm busy!" Sherlock was not at all liking the feeling that he assumed everyone else got when he took them for a ride.
"Well, it's a very unusual name, as I am sure you would agree…"
"John…" came a warning.
"And, frankly, I'm surprised you never wondered…"
"John!" Sherlock cried in exasperation.
"Your name means "blonde locks". I looked it up. Is there some sort of story behind it or..?"
"Wait. Just hold on a tick. You're telling me…that you interrupted my work to say…that I ought to be blonde?" Sherlock hissed, voice dripping with annoyance.
"Well, yeah, I guess so. There had to be some reason behind your name."
"John?"
"Yeah?"
"Shut up."
And, astutely judging the state of Sherlock's temper, John did just that, and promptly got the hell out of the flat. Maybe he'd ask Mycroft.