His experiment starts the next day.
He starts with the physical. He knows it's a long shot, but hey, maybe it could work.
Obviously, even vengeful, mean John isn't bad enough to severely hurt Sherlock himself. Especially not anything lasting.
But he may or may not have been willing to put him into situations that could get him hurt. I mean, they do that all the time anyway with their hobby of chasing dangerous criminals all over creation.
So the next day when Sherlock announces that a certain Harry Curtis needs to be caught and apprehended, and the case inevitably leads to a man-hunt, John's already got a plan in mind.
Basically, he's clumsy. He starts purposefully shoving into Sherlock and making it look like an accident, hoping to shove Sherlock into something hard enough that it really hurt.
John knew it wasn't likely, but he never truly appreciated how graceful Sherlock was until then. Sherlock was too quick, mentally and physically, to be thwarted like that. He'd skitter out of John's way every time, looking exasperated, but mostly saying nothing about it. Sherlock expects John to be stupid, for the most part. That certainly helps John's chances in not getting found out in this whole endeavour, even if it is unendingly annoying.
But he finds quickly that hurting him inadvertently isn't going to work. John is sure that he can't purposefully hurt Sherlock. So pain is struck off the list quickly.
His next one is also pretty dumb, but he figures it's a possibility.
Onions.
So John starts making dinner, and calls to Sherlock passive aggressively about totally not needing help. Sherlock moans as loudly as he can, but still gets up and comes to the kitchen.
"Don't look so put out. I'm giving you a knife."
Sherlock's brow goes up. "You've mentioned on several occasions that you don't like the idea of me with a knife."
"Yeah, well, that onion needs cutting, so I'll get over it."
Sherlock rolls his eyes, but nonetheless goes over to the onion, wields the knife, and gets to chopping.
And even if Sherlock doesn't cry, he decides that this whole thing will have been worth it anyhow, just to watch Sherlock try to cut a damn onion.
Sherlock, being the master of so many things, is spectacularly ignorant on things that normal people can do with ease. Such as naming the planets in the solar system, getting groceries, and apparently cutting onions.
He's looking fairly casual about it at first, but then after a minute of investigating the onion at several angles, he's particularly nonplussed.
"Erm… Sherlock, need a hand?"
"Of course not. I'm merely determining the best course of action."
John raises a brow at him. "What, you've never cut an onion before?"
Sherlock glares over to John with a look he's seen a million times before, the 'your stupid questions tire me' face.
"Need me to give you an easier job, princess?"
Sherlock glares harder. "I'll be just fine, John."
John gestures to the onion. "Alright then. Go on."
Sherlock looks just barely embarrassed. "Don't you have something else to prepare?"
"Something better to do than watch you make a fool of yourself? Not in a million years."
Sherlock rolls his eyes, but gets back to the onion. He tilts his head, looking at the knife.
Part of John wants to give him advice, because he feels so sorry for the poor, helpless man. At least to peel the thing.
But most of him wants to watch his flat mate suffer and giggle maniacally as he did.
But Sherlock does, in fact, figure the peel thing out. Once he picks up the onion to take a look at it from yet another angle, he feels the consistency and seems to decide that any onion he's ever eaten doesn't feel like that. He starts peeling it off into the trashcan and John bites his lip to keep from laughing.
"What?" Sherlock demands. "I don't have to help you, you know."
"You're right. You don't. And then you'll prove that all you need to thwart the great Sherlock Holmes is an onion."
If Sherlock's ever going to go bad and start killing people, it's now, by the look on his face, because he really looks ready to kill John.
But John's right, of course. Sherlock looks back to the onion, finishing peeling it, and then picks up the knife again. Poises the onion. Prepares to make incision. Brings the knife down on the center.
John begins to applaud when the thing is in two pieces, and he didn't know it was possible for the look on Sherlock's face to get meaner, but it does. And John just wants to laugh.
But Sherlock gets back to the onion and does a few more slices. And a few more. He gets more confident. It doesn't look good, but he's succeeding.
And maybe, in hindsight, John should have thought of the next thing that happened as karma. But he's too shocked to consider that this was ironically what he had asked to happen.
Because Sherlock brings the knife down on his finger next.
Sherlock hisses and drops the knife to the ground in surprise.
"Sherlock?" John asks frantically, coming over to Sherlock quickly to look at the wound. It's deep, but not particularly severe, as it didn't get through the bone or anything. The knife was sharp enough that it's a clean slice. It probably stings pretty good, with the onion juice in it.
And of course, John is concerned, and not at all glad this has happened to Sherlock.
But that doesn't mean he doesn't check to see if Sherlock's got tears in his eyes anyhow.
And no. No he doesn't. He's looking down at his sliced finger in frustration, but no tears.
Yeah. John didn't count on this.
"Want me to take you to hospital, or should I stitch it up myself?" Sherlock gives him that 'you're stupid' look again. John rolls his eyes. "I'll get my kit."