There are many words that can be used to describe the interior conditions of the Exterior, but by far the most accurate is 'clean'. Every surface of every room in the thousands upon thousands of miles that comprised the home of the Outsiders was kept in an immaculately spotless state by a mechanism that wasn't fully understood even after decades of study and research.

The Machinist's Shop, however, seemed oddly exempt to this rule. While the walls outside retained their perpetual off-blue sheen common to the rest of the Exterior, every surface inside the Shop hosted come sort of blemish or unseemly contamination, ranging from simple scorch marks to oil and lubricant spatters, dents, and large embedded fragments of metal.

The latter, Pinkie noticed, was a rather distressingly common feature as she looked around the room, trying her very best to avoid letting her gaze fall upon The Machinist's latest attempt at engineering for fear that the... whatever it was would sense her unease and attack.

"Mac, this is a bad idea"

"Eeeyup"

"You know this is a bad idea?"

"Eeeyup"

"And yet you're still doing it?"

"Eeeyup"

"...why?"

Mac turned and fixed his erstwhile 'assistant' with a flat gaze. Soot-covered goggles sat on his face, obscuring his eyes from her view, but she had a fair enough idea of what his expression was underneath.

"Fine, you want to blow yourself up, you can do it yourself. I'm getting food."

"Nope"

Pinkie stopped and groaned, rubbing her face with a hoof. Her eyes hurt. Her hooves hurt. Her flanks hurt. Her entire coat felt rigid and dry, owing to the fine layer of soot that covered every inch of her body, shifting her normally-vibrant pink coat to a more subdued off-grey. Her mane, normally poofy and in constant motion, had been cemented in place, solidifying into the shape it had occupied fifteen minutes ago when Mac's previous 'experiment' had self-destructed.

"Why did I volunteer for this?"

Mac simply shrugged, turning back his latest project. As far as Pinkie could tell, it was a cylindrical arrangement of piping and valves, roughly three feet long and half that around. A tapered bell marked an obvious 'business end', with the opposite end of the contraption filled with thin blades in a sophisticated mockery of a blender.

Pinkie's first instinct upon seeing it was to run. The front was unsafe, the rear was unsafe, and the rest of it was unsafe if only because it was Mac who had built it. It was imminent existential threat to anyone residing in the same city. She immediately understood why Mac's previous assistant had transferred to Operations, claiming that 'Getting shot at is less dangerous'.

"I swear to Celestia, you're going to be the death of me" She moaned, trotting back behind the woefully insufficient blast shield and lowering her goggles, bracing herself for the moment when it was all going to blow up in their faces. "Do it."

"Eeeyup" Mac obliged, depressing his hoof on the Big Red Button.

Nothing happened.

"Uh... Mac?"

Mac held out a hoof, silencing her.

The machine began to whine, quietly at first, but quickly gaining pitch and volume. Mac flipped a pair of switches marked 'fuel flow' and 'igniters', causing it to click ominously. Pinkie backed away, interspersing Mac between herself and the device as a measure of extra protection.

She opened her mouth to ask another question, only to have it snatched from her mouth as the machine let out an almighty roar, a gout of flame spewing from the exhaust. Mac flipped the igniter switch back off and grinned contentedly as the roar grew progressively louder.

Pinkie gaped at the noise, entirely awestruck.

"..." Mac said, his words snatched away by the phenomenal noise of the machine, reaching his hoof towards a guarded switch marked 'reheat'.

"WHAT?" Pinkie screamed, barely able to her own words over the noise, let alone Mac's.

He flicked the switch.

The world became nothing but noise as the roar intensified to an almost painful level. A single solid flame formed, stretching away from the exhaust.  Pinkie smirked despite herself, marvelling that the machine hadn't yet managed to explode.

Mac leaned back onto his haunches, grinning. "..."

"..." Pinkie replied, the noise having grown to such a level that she could no longer hear herself talk.

He gestured to a fourth, much smaller switch on the control panel, surrounded by hazard tape. Pinkie's pupils contracted as he moved to toggle the switch from its current position, marked simply as 'Magic', to its opposite extent marked similarly with 'More Magic'.

Her brain had barely enough time to process what was going on before Mac toggled the switch. The giant flame currently held in place by the machine instantly tripled in length, its previous muted red colour shifting to a dangerous blue-purple hue. The screech of tearing metal was barely audible over the all-encompassing roar of the machine.

She turned to run a second too late. Overstressed by the absurd levels of thrust, the machine tore free of its mountings and accelerated forward. The fuel line quickly snapped taught, swinging it around in a wide arc. It too soon failed, sending the errant machine hurtling towards them, trailing kerosene and flames.

It sailed over their heads, still accelerating. Pinkie felt a rough tug at the back of her head as her mane caught on an exposed pipeline, dragging her along as the freed engine sped towards the access door for the Machinist's Shop.

Pinkie Pie screamed in abject terror as she became the first pony to break the sound barrier indoors.