A/N: So, first of all, I did a bad thing. I'm sorry. I changed the molarity of the HCl in the last chapter. I didn't really think it through before I posted it, and I realized that I needed to make it weaker for plot reasons.

Secondly, I apologize for this chapter taking forever. Unlike Sherlock, I'm not a chemistry major (thank god), but I am a Molecular and Cellular Biology major with ADHD and in the honors program, so between work for my Genetics course, Organic Chem course, Cellular Biology course, Physics course, and some 1 credit hour course on how to do science, my extra honors projects, and forgetting to do all that work, I don't really have that much time and attention span left for writing.

Third, if this is absolute shit, it's because I wrote it at 4am while eating curry in my dorm room while my roommate is gone. Sorry!

But hey, at least you get some of John's POV in this one

NOTE: There is a very very slight trigger warning for eating disorders in this chapter. It's brief and mild as these things go, but if you think there's a possibility that you might be triggered, please look after yourself and don't read this if it could harm you.

"Shit," he muttered under his breath as he watched the bumbling idiot trip forward. He saw the solution from the beaker flying towards his torso, noting dryly to himself that nerve impulses only travel at about 100 meters per second, not even counting the time it takes to actually move, in the second during which he realized that he could not move out of the way quickly enough.

A second later, his shirt was wet and clinging to his skin, though that sensation was eclipsed by his lab partner's sudden scream. He himself should be screaming for help, but he was too busy thinking about what a moron his partner was.

He watched as Dr. Anderson turned to find the source of the offending noise, and couldn't help but find the shocked look on that smug face somewhat comical, but within seconds the professor had schooled his expression back to its normal sneer. Still, the man had to maintain a semblance of professionalism, no matter how much, as Sherlock was sure he did, he would rather watch Sherlock disintegrate into a puddle of bones. Or, more accurately in this case, end up with a delightful chemical burn on his chest.

Dr. Anderson gestured quickly first at John and then to Sherlock, as if to say, "you take care of him," and then ordered all the other students, who were by this point staring, to follow him into the hallway.

After what seemed like an eternity, though it was probably more like 30 seconds, the room was empty, save John and Sherlock, neither of whom had yet spoken a word.

The eye contact was agony. Sherlock wanted to tell John to go join the rest of him because he could handle the situation himself, and probably knew more about the subject of prolonged acid on skin than their moronic professor. He'd run a few experiments on non-human animals, but he imagined the effects were the same.

So Sherlock told him so, minus the experiments part, because he figured the conversation would take more time than he had before the already warm wet patch on his skin started to burn.

"John, I can handle this. I know where the shower is and I know the protocol. Please leave with the rest of them."

"Sherlock, I don't have time to argue. It's my job and if you know the protocol then you know I have to stay with you."

As he said this, Sherlock was already making his way to the emergency shower quickly. He pulled the chain, and gallons of water started pouring down on him.

John tried not to notice the way that the soaked clothes clung to Sherlock's body. His appreciation of the form, however, was stymied by the realization of just how skinny his student was. Yes, there was some muscle visible under the long sleeves of the drenched shirt, but hipbones and ribs could now be seen through the fabric.

Still, this was a crisis situation and he'd have time to reflect on that later. For now, his training kicked in; the acid had spilled on the shirt and it needed to be completely washed away from the skin. Sherlock should know that, so why hadn't he taken his shirt off before anything else?

"Sherlock, you need to remove your shirt."

What happened next was something John didn't expect. His usually cocky student looked him straight in the eyes, and something flickered across his face that looked like vulnerability. If it had been any other student he would have said there was a hint of desperation in the word "please," as it fell softly from Sherlock's lips, its effects augmented by his shivering under the freezing water.

John knew he didn't have time for dealing with modesty when, by this point, the acid must have started burning his student's skin. Fuck protocol, he had get Sherlock to wash the acid off his skin.

"Alright, I'm turning around and giving you privacy. For fuck's sake, Sherlock, why is keeping your shirt on more important to you than avoiding acid burns?"

The only response he got was the sound of wet clothing being removed from behind him.

John kept his curiosity to himself. If this were a female student he would understand, by why was Sherlock behaving this way?

Sherlock himself wasn't sure why he cared so much if John knew. Half of campus had seen his scars. It wasn't as if he tried to hide them. Maybe some small part of him wanted John to like him. John interacted with him as one knowledgeable person to another, almost like a friend. He didn't have friends, though. Still, the idea of John seeing him as the pathetic human being he really was, speaking down to him with pity or concern, twisted knots in his stomach. Best not analyze that one too much…

They stood there with only the sound of water pouring out for another fifteen minutes before Sherlock finally spoke.

"The acid is gone by now. It's a minor burn, but it shouldn't leave lasting damage. Is there anything I can put on now, since my shirt is contaminated?"

John thought for a second before replying. "I think I have a spare T-shirt in the back room. Give me a second and I'll grab it."

"Um, is there anything with… long sleeves?" the voice behind him asked. And dammit, there was that unexpected note of vulnerability in his student's voice again.

Come to think of it, John had never seen Sherlock wear anything other than long sleeves. Maybe he had embarrassing tattoos? Or got cold easily, which would be understandable after 15 minutes under that freezing water? Or anything other than shooting up. It had to be something other than that.

"Sherlock?"

Sherlock felt his heart pound in his chest. It was stupid to ask for long sleeves. It gave too much away. He hadn't had a choice though. Now John was going to know. Not that it mattered. Why did he fucking care? He shouldn't care. It's not like John could call his parents or something equally ridiculous. Why the fuck did he care?

"Yes?"

Another pause

"Do you trust me?"

"I don't trust anyone."

That was unexpected. John couldn't even bring himself to think about the implications of that statement right now.

"I see."

Another pause

Then John continued, "All I have is a T-shirt. How about I grab it for you and then we take the side door out to the TA office. I'm the only one in for the next few hours, so no one will see you. I'm sure we can find you a shirt somewhere."

Sherlock didn't really care if anyone else saw his arms. Sure, he didn't want Dr. Anderson to have the satisfaction, but half the class had seen his arms during other classes. He couldn't exactly tell John this though, given how hard he'd tried to keep him from seeing. There was no other choice.

"Fine. Just, don't say anything, okay?"

He watched as John nodded, probably assuming that Sherlock didn't want anyone else to know about this mysterious secret, not realizing that Sherlock didn't care if others knew, he just couldn't handle what John would say when he saw the scars.

John slowly began to turn around.

Sherlock said nothing to stop him. The part of him that tabulated possible outcomes and calculated probabilities had known this would probably happen from the second his lab partner's beaker had tipped towards his shirt, but now it was real. There was nothing he could do to change the outcome.

That still didn't stop the inexplicable sting he felt as he watched John's face flash with realization and then something he couldn't quite put his finger on.

Maybe it was because he didn't see much of it at all during his childhood, but Sherlock couldn't quite identify the look on John's face as he realized that it wasn't heroin that Sherlock had been hiding, but rather self-inflicted damage for no purpose other than that damage. The look on John's face as he realized that Sherlock, who seemingly thought more highly of himself than any other mortal, cared little enough about himself to carve those scars by his own hand – that look was compassion.