John has skin like a snake. First John would do it himself with a lot of problems and sometimes causing patches to bleed since he didn’t always have time to do it while being in the army. Since moving in with Sherlock John never had that happen.
In a surprising way that John never though he would find someone who would even think of doing this for him, the consulting detective even stopped once in the middle of a case to gently rub the skin slowly until it fell off painlessly. Or if John had a stubborn patch, Sherlock would make a water bottle appear out of his pockets and moisten his own scarf and dab at John’s skin.
He sheds his skin, but someone clueless doesn’t know. One day this person yanks on John’ skin causing it to tear painfully since there was only a small amount loose.
