an experiment: 2.

You shouldn't let him use you. That's all he's doing; using you, for an experiment. Just another stupid experiment like the head in the fridge. There is no emotion here, John. You have to stop.
Before you get hurt.

Sherlock's fingers hooked inside John's bottom lip, and pulled.  John barely noticed until the pressure started. Nails scraping his gums, forcing into the gaps between his teeth until his jaws were pried apart. Not that it took much prying.

Sherlock's fingers tasted like chemicals and blood and dust sweat and something else. Something odd.  But good. Most definitely very good.

Before John barely had time to notice, Sherlock's mouth had left his to be replaced by the aforementioned fingers. Thumb holding his chin, the fingers toyed with his tongue for several delicious  seconds before he remembered himself, and stopped.

Which certainly isn't to say Sherlock stopped. No, he just kept kissing and kissing until John's mouth was bruised and suddenly they were both in the armchair. Sherlock slid into the doctor's lap and his hands were everywhere and then…