Rating/Content: PG-13. Google-sourced UK shopping experiences. Customer service. References to gore. Humour and silliness.
Disclaimer: I did not create, nor do I own these characters or their world. I have also never been in a Tesco, but I am certain that their staff are unfailingly friendly and cheerful at all times and that every single one of their stores is a wonderland of retail delights.
Notes: I bought a microwave last August after my own went pop and had this scenario running around in my head the whole time I was in Walmart. It took a while to pull together but the Mini_Wrimo Challenge helped with that significantly, so here it is.

Summary: John and Sherlock buy a microwave.

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Variations in Purchasing Criteria
by Caffienekitty
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Squinting around the brightly-lit and colourful Tesco, Sherlock's upper lip twitched and he made a noise like he'd just stepped in dog vomit.

"It's a Tesco, Sherlock," John said. "Not the Fourth Circle of Hell."

"The electronic orchestral rendering of 'Girl from Ipanema' would beg to differ."

John heaved an aggrieved sigh, then marched off in the direction of Home Electrical, leaving Sherlock to slouch along in his wake like a sulky teenager.

"It's your own fault we're here, you know," John said over his shoulder. "You're the one who killed the bloody thing."

"It 'died' for a case, John. They'd never have proven that the explosion wasn't an accident so quickly if I hadn't-"

"Destroyed the bloody microwave in the name of science, yes, I know. D'you think Mrs. Hudson likes having her tenants blow up her appliances?"

Close behind John, Sherlock sniffed. "Mrs. Hudson understands."

"Mrs Hudson doesn't understand, Sherlock, Mrs. Hudson tolerates." John huddled down into his jacket, glancing down aisles full of retirees and mid-afternoon mums as they passed them. "God only knows why."

Sherlock said nothing but John could feel his flatmate's waves of smugness through the back of his head. Insufferable prat.

They turned down an aisle to face a row of gleaming microwaves. "Look, let's just get the cheapest one and leave, then your delicate sensibilities won't have to suffer the torturing of the 'Girl from Ipanema' any longer."

"It's changed. Now they're inflicting unaltered One Direction on us. I suggest fleeing immediately."

"As soon as we get a microwave."

"Oh, of course." Sherlock oozed mock fervour. "We've come too far to turn back now!"

John sighed, again; he expected he'd be sighing a lot more before the day was out. He went to the display model with the lowest price tag and opened the door. "Right, looks good, let's get one and go."

"Not big enough," Sherlock muttered.

"What?"

"Not big enough. I could never fit an entire human head in that."

John flinched, looked around in alarm and hissed, "Sherlock!"

"What?" Sherlock innocently examined a display of black and silver Kenwoods. "You insisted I come with you to this loathsome place so that I could be certain the new microwave would meet my needs."

"And you need to microwave human heads?"

Sherlock looked mildly wounded. "I may. In future."

John groaned and hid his face in one hand. "Okay. Fine." He stepped to the next least expensive model.

"Still not big enough."

"An adult human head averages about five litres in volume, Sherlock. You of all people must know that. A seventeen litre volume microwave is plenty for... anything you'd need to do."

"No. It must have sufficient space for the head to freely rotate."

John would later insist that he did not squeak indignantly at that point. Rather than pull together a reasoned diatribe on the thought of some poor bastard's severed head rotating away in their microwave or anyone else's, he went the health and safety route; a familiar one which he'd followed both under live fire and while dripping with someone else's mucous (on one memorable occasion, simultaneously).

"Well then." John crossed his arms. "We'll need to get two."

"Why?"

"Because if you're microwaving someone's head in one of them, I'm not going to be using it to heat anything we're going to eat. It's not sanitary."

Sherlock didn't quite pout. "You had no qualms about using our old microwave after the eyeballs."

"The eyeballs were in a jar, Sherlock!"

Suddenly, John felt like they were being watched. He looked over his shoulder, saw nothing, but then caught a motion closer to the floor and turned with slow and creeping dread.

A child of about four stood there, big-eyed, clutching a disreputable stuffed rabbit.

"Where's th'eyeballs?" the boy queried before jamming his thumb into his mouth.

John glared at Sherlock who rolled his eyes. With yet another sigh, John crouched next to the little boy. "It's all right. There's no eyeballs, we're just making up stories."

"Oh," the boy mumbled around his mouthful of thumb.

"Anton?" A young woman holding a smartphone and a shopping basket peered around the corner of the aisle.

John stood up, "Um. Hello, missing someone?"

"Fine thanks," the woman said, not paying attention as she shifted her basket and phone to tug on the little boy's shoulder. "Anton, come away, you're not to wander off. And thumbs out."

The boy pulled his thumb from his mouth with a pop and a scowl. "But I wanna see eyeballs inna jar!"

"Maybe later pet, if you keep your thumb out of your mouth," the young woman said, towing the child away, focus already re-absorbed by the shopping list or whatever it was on her phone.

Watching them go, John emitted a high-pitched noise that might have been half a giggle. "That was close. Sherlock, really, we have to-" John turned around to find an absence of sullen lurking coat. "Sherlock?"

Sherlock was further down the microwave display, looming over a bored twenty-something with a name-tag and a red-checked shirt.

"Oh god." John hurried over in hopes of preventing actual psychological damage to the unsuspecting store employee.

"This one's on offer this week, thirty-five quid, fantastic," the young staffer said, sounding about as enthusiastic as a rainy Tuesday.

"It's only twenty litres."

"Fits most things you'd put in a microwave." The blank-faced clerk snapped her gum.

"Not the things I put in microwaves."

John butted in. "Hi, ah, excuse us a second."

"Whatever you like," said the indifferent clerk.

John smiled, nodded and grabbed Sherlock's elbow, towing him out of earshot.

"Can we please not get thrown out of Tesco for planning an appliance purchase around its future use-"

"Potential future use."

"Fine, potential future use in desecrating severed human heads?"

"Don't be ridiculous, John. Most human heads aren't consecrated in the first place, so they couldn't be 'desecrated'. Though if someone murdered a member of the clergy..."

It was indeed John Watson's day for frustrated sighing. To avoid further speculation on the likelihood of various religious officials being killed by having their heads microwaved, John changed conversational tack. "So. What exactly is wrong with a twenty litre?"

"The neck stump would catch as it rotates."

John felt his face stall. "Really."

Sherlock smirked. "No."

"Good."

"I'd have to remove the tray for a head to fit in a twenty litre microwave. It wouldn't rotate at all," Sherlock said as though discussing the weather. "It's thirty litres or nothing, John."

There wasn't a deep enough sigh in the world to express John's exasperation. He threw his hands in the air in surrender. "Fine! Whatever! I suppose we'll get use out of it in the meantime, and just hope no one microwaves anyone's head."

"You do realise the more we discuss it the more I want to try it?"

"What? No! No bringing home a head to microwave just for kicks!"

Sherlock looked affronted. "For science, John, not for kicks."

"There will be no spurious microwaving of any heads, for science or otherwise," John decreed. "If it will solve a crime or catch a murderer, fine."

Sherlock opened his mouth.

John held a finger up. "A current, active crime, not something from one of your books or a cold case."

"You're in luck there," Sherlock said wryly. "I doubt many cases prior to the 1980's would involve a microwaved human head at all."

John sighed, yet again, starting to wonder if all the sighing was going to make him go hypoxic, and counted off on his fingers. "One, it must be an active crime. Two, there must be a danger of the murderer getting off or repeating the murder before alternate forms of analysis can get the same data. And three, there must be no other equivalent options available at all, anywhere, like- like microwaving a pig's head instead."

"Nonsense. A pig's skull would never fit in a household microwave. Too long. The snout would be far too protuberant."

John pinched the bridge of his nose, feeling a headache coming on. Probably from all the sighing. "Fine. If any of those situations arise, you will pack up the appropriate head and the microwave, take them both down to Bart's, do it in the lab there, and we will get a new microwave."

"Lab. Of course, excellent. Better to assess cell damage."

John nodded encouragingly. "Not to mention you'd likely need to match a specific model if someone's murdered by having their head microwaved anyway."

"Yes..." Sherlock's eyes glinted as they examined the row of gleaming kitchen appliances with a new zeal.

"So... then we won't really need one that can do a whole head right now, will we?"

Sherlock shrugged eloquently and continued scrutinizing microwaves with the beginnings of unholy glee.

John sighed for what he hoped was a final time on this shopping trip, returning to the clerk who tucked away her mobile as he approached. "Hi, yes, we'll have the twenty litre model."

"Brilliant." Snap went her gum. "Black, white or silver?"

"Doesn't matter- actually, no. God. Black." This microwave would be facing future stains unheard of in any other kitchen.

The clerk patted one of the boxed microwaves under the display. "Did you need any help carrying that today?"

"No, thanks, that's fine." John bent and grappled the unwieldy box. It was heavier than it looked, and awkward.

"You're sure about that? I can fetch you a trolley."

"I'll manage, thanks," John said through gritted teeth. The clerk shrugged and drifted away as John shifted the sizable box to a more stable grip.

Sherlock very helpfully stood and watched, gloved hands clasped behind his back. The strains of an easy-listening instrumental rendition of 'You Shook Me All Night Long' drifted placidly through the store.

"Right," John grunted under the load, nodding in the direction of the checkouts. "We have a microwave, we can go now. Happy?"

"Well." Sherlock rocked back on his heels. "Since we're here regardless, I'd also like to look at the blenders."

"What? Why would we need-"

With a smug wink, Sherlock spun and strode down the nearby Small Kitchen Appliances aisle.

"Oh, no. No! No pureeing anyone's organs in our kitchen!" squawked John, lugging the microwave and waddling rapidly after his flatmate. "Oof. Sherlock!"

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(that's all)