When they first met, Sherlock called the limp psychosomatic. He was right, of course, and it has gotten better. But the mind is a powerful thing; it holds on to pain with a vengeance. As much as it bothers him, frustrates him, maddens him, John Watson still feels pain in the perfectly intact flesh of his leg. No amount of adventure will make it go away completely.

And John's frustration frustrates Sherlock. Too much angst hangs in the air, makes the flat all muddy with stupid thoughts (only Sherlock is allowed to truly mope). All of John's nerve endings are fine. And Sherlock is going to prove it.

He starts where John tells him the pain is worst. His lips are dry and cool as they close lightly over the skin, somewhere between a kiss and a toothless bite. John shudders on top of the bed. One motion and Sherlock's theory is proven, but he doesn't stop. This is also a psychological experiment: countering psychosomatic pain with purely somatic pleasure, rewriting the responses of the nerves from the outside. His hands cradle John's leg in place as he drags his lips across it again. John's hands curl into the bed sheets. His face looks attentive but not tense, and he promised to tell Sherlock if it hurt, so Sherlock continues.

Down the thigh, over the knee cap, lifting the leg to get to the back of the joint, then down the calf muscle. The kisses grow warmer and wetter as he goes, his own physical responses working in tandem with John's twitching muscles. A long lick from the ankle back up to the knee has John breathing heavily in his throat. Sherlock presses his fingers into the starting spot, the supposed focal point of the pain, and John snorts: half laugh, half impatience.

"Stop doing that and get up here."

"You are a terrible test subject."

"I'm not a petri dish. So get up here and kiss me before I join you down there."