John had an irrational fear of Sherlock's chemistry experiments. That's how he saw it anyway.
There was only that one time with a fire... and to be fair, John did not get hurt, only Sherlock. And the curtains. And a little bit of the table...
But John really had no reason to be worried. Sherlock had an excellent grasp of chemistry. The only reason the last experiment had gotten a little out of hand was because John insisted upon distracting him with that telly show he so enjoyed. It was rather illogical. A box that was bigger on the inside? Sherlock had been busy pointing out all the inaccuracies when the flame on his Bunsen burner got out of hand and attacked the curtains. ("They were rather dreadful," Sherlock had pointed out later. John seemed to disagree.) So really, as long as John didn't distract Sherlock while in the midst of an experiment, both the new curtains and John would be fine.
That's what he thought anyway.
It was an experiment that he had done back in... oh it must have been primary school. Probably not in class, but something on the side, his sciences teacher was excellent, let him do experiments while he watched.
This one was rather entertaining, although not very educational. He was preparing to watch the reactions of alkali metals with water. He started with lithium. Not very impressive, but fun. It spun and fizzed around the beaker. He moved onto sodium. More of the same. Potassium was slightly more interesting. It caught on fire. Fascinating. Science was entirely fascinating. Rubidium and caesium were both better than the last.
It was only when he was ready to move onto francium that John seemed to finally realize Sherlock was doing something that might not be entirely safe and came to ruin the fun.
John took one look at Sherlock, decked out in his full lab gear, goggles, gloves, and his ridiculous apron, and said, flat out "NO."
But Sherlock was extremely focused, and had not heard John coming up behind him.
It was a little late. Sherlock was already holding the francium over the beaker with his tongs. And John startled him. It shouldn't have been such a big deal. A tiny sliver. Smaller than all the others. He should have been able to extrapolate the extent of this reaction based on the previous ones. (To be fair, when he did this experiment so many years ago, his teacher has never let him go past rubidium. He now understood why.)
The reaction was bigger than Sherlock would have predicted. The beaker shattered, sending bits of glass everywhere. At John. Into John.
Bet you don't think my apron is so ridiculous now do you?
Because, there was indeed glass shards embedded in the apron, dangerously close to Sherlock's skin.
John was not so lucky. Sherlock looked up. There was blood. John. John's blood.
Oh dear oh dear oh dear.
Sherlock paled. He had hurt John. Again. (John wearing a bomb, John kidnapped by Chinese smugglers).
John swore. "Sherlock!"
He left the kitchen, carefully, not stepping on any of the glass shards in his sock feet. Sherlock followed him like a puppy. He distantly noted the John was getting blood everywhere. Mrs Hudson would not be pleased. He followed John into the bathroom, where he was shedding his jumper. Sherlock looked him over. Multiple small shards in his abdomen and chest, none seem to be very deep, nothing life threatening. Nothing needing stitches. Sherlock was relieved. Until he saw John's left arm anyway.
The glass embedded there was larger than all the others. It was leaking steadily (not pumping though, that's good, no arterial involvement) and would need stitches.
John was clumsily trying to dig the first aid kit out from under the sink with one arm, holding his bleeding one to his chest with a towel in between. Clever. Sherlock pushed him aside and pointed to the toilet. "Sit."
Sherlock admired the neatly organized and stocked first aid kit for a moment before pulling out the necessary supplies. John had obviously learned after the first time Sherlock had hurt himself in the flat, and was astounded to hear that Sherlock had no sort of first aid kit. Or even plasters.
Sherlock began carefully pulling out the little shards of glass stuck in John's abdomen and chest with the tweezers, carefully disposing of the glass and cleaning each wound and covering them individually with gauze. He could tell the antiseptic was hurting John, but he barely showed it. The brave soldier.
He did this silently, but he could hear John thinking, yelling rather, at him.
When the only wound left was the one in John's arm, he pulled the towel away and inspected it.
"Needs stitches," he remarked.
John only nodded wearily.
John eyed him.
Sherlock sighed, "Do I need to spell it out for you?"
John still looked a bit stunned.
"Oh, stop that. You haven't lost enough blood to be in shock, so start thinking. I can hear you doing it. Just the wrong topic."
A look of realization came across John's face. He looked horrified.
"No," he said firmly. "You are not stitching me up."
"You are left handed. The wound is in your left arm. Although you are somewhat ambidextrous, not enough so that you would be able to stitch it up yourself." He frowned. "Not properly anyway. Do you really want to go to A&E, sit there for hours, have to explain how you got it, and have some med student do a hack job?"
John scowled. But Sherlock could tell he was considering it.
"I've watched you sew me up enough times."
John hesitated, not quite sure.
"I'll do it exactly as you tell me."
John relented. "Fine, but if you screw it up I am binning every single one of your experiments."
Sherlock beamed and set about drawing up the local anaesthetic.
He injected it carefully and just as John instructed. He prepared sutures precisely as John told him. And he stitched up the wound to the letter, exactly as John explained it.
And when he was finished, he sat back and admired his work. Even John grudgingly admitted that it looked good.
"Better than a med student would do," Sherlock remarked.
John nodded. "Don't think you're off the hook though. Go clean it up," he ordered.
Sherlock scowled, but obeyed, muttering the entire time. He didn't want John to mistakenly believe that he was fine with this. But he had hurt John, and he felt guilty.
So he binned the shards of glass, and wiped up the spill, and even left the kitchen tidier than it was before. He made a cup of tea for John and delivered it to him where he sat on the couch, watching that show with an undoctor again. John eyed it suspiciously.
"It's tea, John. It's 'sorry' tea." He spat the words out. They tasted funny. "But you can't just interrupt an experiment without wearing protective equipment. I hope you learned your lesson," he sniffed.
But within the next couple of days, another apron appeared, hung next to Sherlock's on the coat rack.