Lips
John
For a man who spent so much time frowning Sherlock's smile was bright enough to illuminate even the darkest room.
The rare phenomenon ran from one ear to the other and stretched his cheeks so his eyes became all small and squinty; it was enough to make John's own lips twist up.
If his flatmate had been a woman John would have had no qualms about kissing that smile right off of her face...sometimes he considered it without the 'if'.
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Sherlock
Sometimes, when he didn't have a case, Sherlock would spend hours watching his flatmate.
John Watson had many curious habits and one that always intrigued Sherlock was his peeking tongue.
That moist, pink tip would poke out whenever he was anxious, ecstatic or thinking especially hard (for a normal peson.) It would flick out dampen his lower lip and then rereat back into his mouth.
Fascinating, Sherlock mulled, absolutely fascinating.