It should have been an excellent day for studying. A cold wind, blowing from the north, had caused the temperature in Ponyville to drop sharply. Ponies throughout the town had retreated into their houses and closed the doors and windows. Summer was over and it was time to start thinking about indoor activities and preparations for winter. A good day to stay indoors with a book – or a stack of books – which was just what I planned to do. Today I would disprove an old mares’ tale that said that there was somehow enough energy locked up in a teaspoon of water to provide all the power Equestria needed for a year. Utter nonsense. I had already drafted a letter to the Ponyville Express and just needed to check a few facts. I had a pile of relevant books stacked neatly on my desk. After a few minutes of staring out of the window, watching the wind shake the outer branches of my tree sending the leaves dancing in the air, I quit day dreaming, opened the first volume in my pile, and started reading:

Energy in Equestria

Life in Equestria is sustained by the energy from Celestia's Sun. Plants absorb this energy and store it. Ponies and other animals get their energy from plants through the food we eat. Without magic, energy can never be created or destroyed...

The sound of somepony hammering a hoof on the door downstairs momentarily disturbed my concentration, but I paid no further attention. Spike could take care of whoever it was. I continued reading.

...While nearly all energy in Equestria originates from the sun, there is an important exception: the energy present in the rocks beneath our feet, which produces heat deep in the core of our planet...

I heard the sound of a kangaroo hopping up the staircase. Actually it was a pony, and I had a pretty good idea which one.

“Hiya Twilight! Whatcha doing? I haven't seen you since yesterday...”

Pinkie Pie.

“...and I was gonna ask you about... Well actually I was gonna ask you loads of things. What's your favourite flavour of... Oh! You're stuck into studying. No worries. I won't bother you. I really just stopped by to ask if you could spare a bit of uranium. Have you got any?”

“Sure,” I answered without paying much attention to what she was saying, “have a look in the kitchen.”

“Okeydokeylokey.”

I looked up at the window and watched the reflection of her pink mane bouncing away, and breathed a sigh of relief. Maybe my afternoon of studying could go ahead as planned. Much as I enjoy Pinkie's company, I do prefer it when she gives some advanced notice before visiting, which lets me properly schedule the time, ensure I have an adequate stock of sugary snacks, and invite Rainbow Dash to come at the same time so when things get too much, I can just leave them planning pranks, and retreat to my bedroom study. I turned back to the page in front of me trying to ignore the noise from downstairs as Pinkie ransacked the kitchen for whatever she was looking for.

...Nuclear processes such as the fission of uranium atoms convert mass into energy, giving off heat...

Wait a moment. Did Pinkie just say 'uranium'?

I spun around to see Pinkie bouncing back into the room with a roll of paper in her mouth. She stopped in the doorway and dropped the scroll, letting it unroll all the way across the floor to my hooves revealing a long list.

“I've been through your kitchen cupboards, refrigerator and freezer and taken a full inventory,” she announced. “They contain: six sacks of oats; two bales of hay; eight pounds of carrots; four bags of apples; fourteen different varieties of tea; a large jar of cookies; thirty-five tubs of ice-cream; eighty-six bars of chocolate; five half-full bottles of maple syrup; some yellow mushy stuff that smelt a bit like peanuts; and twelve bottles of some really weird looking fluorescent liquid labelled 'experimental sample #01' to 'experimental sample #12'. I also found a badly stained copy of 'Magical Mysteries and Practical Potions'; a textbook on extragalactic astronomy; and a dictionary of griffin dialects.”

She looked up, having reached the end of the list. “But no uranium. Are you sure you keep it in your kitchen? That's not really a sensible place to store toxic material.”

“Sorry Pinkie,” I mumbled as my brain tried to process all the information it had been thrown. Must remember to ask Spike to tidy up the kitchen. “You want some... uranium?”

“Yeah, I need some five percent enriched uranium fuel pellets, or mixed oxides will do just as well.”

“What?” I said.

“Mixed uranium and plutonium oxide fuel.”

“No... I mean... What do you want it for?”

She rolled her eyes at me. This was evidently a stupid question.

“To make a sustained nuclear chain reaction. What else do you do with it?”

I stared back at her, unsure what to say. “Err... I'm sorry Pinkie, I don't have any uranium or plutonium.” Should I be apologising for that? Was it remiss of me to not have it in stock?

“Ah, too bad. I'll just nip out to the rock farm and get some more. I just thought given all the cool science stuff you have in your basement, you might have some. But, no problem, see you around. La-la-la-la-la.”

Pinkie skipped off down the stairs leaving me staring at the wall trying to get my thoughts in some sort of order: Pinkie Pie, uranium, nuclear chain reaction. This was not the first time I had been abruptly left in a state of complete bewilderment by my fuchsia-maned friend. In fact, on this occasion, Pinkie's explanation actually followed a logic I could recognise, even if it didn't add up to any sense whatsoever.

I suddenly became aware that a little dragon was standing in the doorway staring at me with an expression of concern.

“Are you okay Twilight?” said Spike, “you've gone kinda pale.”

I turned to face him. “Pinkie Pie said she needed some uranium,” I said.

“She must be baking a new flavour of cupcake,” he replied, licking his lips.

“Uranium is a radioactive heavy metal,” I shouted, “it's highly toxic!” It occurred to me as I said this that it might not be true for dragons. “And she said she wanted to make a nuclear chain reaction!”

“She's just being Pinkie Pie,” he said calmly.

He was right. We had been here before. And at this point the usual solution was to say 'It's just Pinkie Pie', pretend that this explained everything, and get back to whatever I was thinking about before, without further ado. Indeed experience had taught me that any other action might lead to random heavy objects dropping on top of my head. However this time, some level of intuition inside my brain was flashing a warning signal that if I ignored this, it might somehow end in Ponyville disappearing under an enormous pink radioactive mushroom cloud.

“Do you know what a nuclear reaction is?” I asked Spike.

“Erm... no.”

“You know the story of the Incantatem Fissio Individuuma?”

“Erm... no.”

“Have you still not read that book about obscure unicorn history that I gave you two years ago?”

“Erm...”

“It was one of the most important events shaping the foundation of the pre-classical era. During the ancient Equestrian wars, Starswirl the Bearded discovered a powerful magic to split atoms of uranium in a nuclear reaction, and thus convert matter into enormous amounts of free energy. He used it to defeat the Dark Shogun Sorcerers who were terrorising the Eastern Empire, but at a terrible cost. Whole towns and cities in Neighpon were laid waste, and the land remained poisoned for years. He swore the spell should never be used again, and never wrote it down.”

I did not mention that although it had never been written down, legend had it that the spell had been passed by word of mouth to his most trusted students. There were many stories about it. Some talked of a later, even more powerful enchantment which used this power to fuse hydrogen atoms together, releasing further energy. There were rumours that Princess Celestia knew it, but I found that difficult to believe. I could not associate my trusted mentor with such dark magic.

My number one assistant just looked at me blankly. I thought back to what I had read about Starswirl's terrible spell. The principle – to split apart atoms and reform them as lighter elements, so that the mass difference would be liberated as energy – was simple, but the magic to manipulate matter at this scale must be formidable. Was Pinkie really dabbling in such dark arts? She couldn't do magic as I knew it, but she had improbable powers of her own. Did she understand the danger? I made a decision: this needed further research.

“Spike!” I yelled, “I'm going out. Keep an eye on the library. And in the meantime, can you clean out the kitchen?”

I wrapped a scarf around my neck and ran out the door. Outside I scanned my field of view and caught sight of Pinkie hopping up a near vertical hillside in the distance with her characteristic gait. Boy, that pony could move fast. I focussed on the moving pink pony, calculating her expected trajectory, then closed my eyes and teleported to the summit of the hill.

It turned out I had pinned down her location more accurately than anticipated. On materialising out in the open countryside, I had a momentary vision of a huge grinning pink face bearing towards me, before we collided and were both sent rolling head over hooves down the hill.

I picked myself up groaning, while Pinkie bounced around me.

“Twilight! Did you find that you did have a stash of uranium hidden under your mattress?”

“Err... no... I... well... erm...”

“You want to see what I'm up to 'cause you're afraid if you don't, it might somehow end in Ponyville disappearing under an enormous pink radioactive mushroom cloud?”

“I... err... You don't mind?”

“'Course not. You can come with me out to the rock farm and meet Mom and Dad and the sisters and Rockie, my pet rock. It'll be totally cool!”

She trotted off and was soon halfway up the next hill. I cantered after her, struggling to keep up. It occurred to me that this was an opportunity to ask a question which I had been meaning to raise for some time. “Err, Pinkie?” I said as soon as I managed to draw level. “What exactly is a rock farm?”

“Duh!” she replied. “A place where you farm rocks of course. I told you about the rock farm where I grew up. You remember? When I told you about the day I got my cutie mark.”

Farming, as an exclusively earth pony activity, had been absent from the curriculum at Princess Celestia's School for Gifted Unicorns. I would readily admit that I was no expert on the subject. Nonetheless, I was a well-read student, and I was acquainted with a fair about of information on the subject from books, and from conversations with our friend Applejack. I was aware that it involved the cultivation of plant crops, such as apples and oats, and the rearing of animals for wool, eggs, milk, and potentially other purposes which ponies did not usually talk about. I couldn't see how rocks could be applied to this model.

“So,” I said, with what I hoped was the right level of sarcasm, “how does that work? You bury little pebbles in the ground and wait for them to grow into big boulders?”

“Nah. That would take too long.”

I decided not to interrogate further; I would find out the answer soon enough. We continued trotting up and down the hills which lay to the north-west of Ponyville. As we reached higher altitudes, the vegetation became sparser, and on the crests the wind was blowing strong, flattening out our manes and making me shiver. Eventually we reached the summit of a rocky outcrop from where we had an excellent view of the next valley.

“There you are,” said Pinkie proudly, “the family rock farm.”

I looked across the panorama. There were hardly any trees or plants. The flat bottom of the valley was strewn with rocks, from gravel to large boulders. Various farm buildings punctuated the landscape, linked by roads. It would be pretty dull, except that every barn, water tank, and many other buildings had been painted in bright vivid colours. Fluorescent murals depicted images of smiling happy ponies prancing under a sky filled with rainbows, streamers and balloons. It was impossible not to smile at this joyous art show.

“You like it!” A very satisfied Pinkie Pie threw her hooves around my neck and hugged me.

“Your artwork?” I asked.

“Yeah,” replied Pinkie. “Come on. You've got to taste Dad's rock cakes.”

We cantered down the hillside and into the farm complex together, past a large yellow and black sign marked: 'Caution Rocks Turning'. The site was larger than I had imagined, and clearly employed a lot of ponies. We passed a number of earth ponies, busy shifting rocks, or engaged in other activities. Of course Pinkie knew everypony and met them all with a smile and greeting. We reached a timber framed farm house with a sagging thatched roof, somewhat older than the other buildings. Pinkie caught sight of a light grey mare with her mane tied up in a bun. She then rushed towards her.

“Pinkie?” cried her mother.

“Hi Mom, what's new? I'm back, and Twilight's come with me, look...”

“P-Princess Twilight?”

“Hello, Cloudy Quartz isn't it?” I said politely, extending a hoof to greet her.

“J-just a moment.”

She darted to one side and into the house, through a wooden door, which she closed behind her. I waited outside with Pinkie, trying not to hear what was being said behind the door, and failing.

“It's Princess Twilight Sparkle. Quick! Clear up this mess. Get out the emergency cakes!”

“She's with Pinkie, it's just a casual visit.”

“There's nothing casual about a visit from royalty!”

I felt conscious of the mess the wind had blown my mane into, and of my faded old scarf (bought from the Canterlot flea market for half a bit). Judging by the sounds coming through the door, Pinkie's family were turning their kitchen upside-down. After a minute or two, Cloudy Quartz opened the door, smiling nervously. “Please come in,” she said. “Take a seat and I'll get you some tea. Help yourself to cakes.”

We walked through the door and into a large kitchen. It was not unlike the kitchen at Sweet Apple Acres, and pretty much how I imagine farmhouse interiors are throughout Equestria. The floor was laid with well-worn flagstones. Along the far wall were granite worktops, and a sink large enough to wash a foal; the other wall supported a wooden dresser stacked with plates, and a glass cabinet full of musical instruments (with a small label: 'Break glass in case of parasprites'). Behind a large table stood Pinkie's family: her mother, Cloudy Quartz; her father, Igneous Rock; and her sisters, Marble Pie and Limestone Pie. I had met them briefly when they visited Ponyville, but never really got to know them. The table was covered with a pretty floral table cloth, and stacked high with cakes, tarts, pastries, sandwiches, buns, cookies, pies and many other sweet treats. There was a porcelain tea service, pots of cream and jam, and teacups and pretty little plates sitting on doilies. It was a tea party fit for a princess. I wondered if I was fit to join it.

Pinkie had no inhibitions. At the sight of the feast, her eyes widened and her grin extended even further than normal around her head. “Awesome!” she cried, and stuck her face straight into an enormous chocolate gateau at the end of the table. I sat down and took a cup of tea and a rock cake.

“Pinkamena,” hissed her mother. “Behave yourself. We have a guest.”

“And you are such a totally super awesome host,” replied Pinkie, sticking her chocolate covered head out of the far side of the cake and grinning gleefully at her mother. She picked up a jam tart with her tongue, dipping it in a pot of cream, before retracting it into her mouth. I took a bite of my rock cake. It was delicious. I tried not to stare at Pinkie and instead smiled nervously at her parents. I wanted to ask what a rock farm was, but I didn't want to be told it was a place where you farm rocks.

“What sort of rocks do you farm here?” I said.

“This is a mixed rock farm,” said Igneous Rock. “We have diversified in recent years.”

“It's the best way,” said Limestone. “Putting all your rocks in one basket is too risky in the present climate.”

“And it's not sustainable in the long term,” added Marble.

“Rocks don't grow on trees,” said Igneous nodding.

“The building stone and pet rock trade is doing well,” said Limestone, “but it's still difficult for us to compete with the glittery imports from the Crystal Empire. The bulk of our business is now nuclear fuels and radioactive waste management.”

I ran this statement through my head. This sounded serious. Pinkie had now finished the chocolate gateau and was now standing with four hooves on the table, and her head in a bowl of trifle, making loud slurping noises. Cloudy smiled at me nervously.

“What does that involve?” I asked.

“Well,” said Limestone, “we take the fertile rocks from the south field, and stack them in a breeder reactor in the east barn, where they are irradiated to convert the fertile elements into fissile uranium and plutonium isotopes. We then break up the rocks, and extract the useful elements, and form them into fuel pellets, which we sell.”

“Then,” continued Marble, “we receive the spent fuel rods from nuclear power plants around Equestria. Once they've cooled down, we break them open, and extract any remaining fuel. Then we take the remaining highly radioactive waste, turn it into glass rocks, encase it in steel, and bury it deep underground.”

“Well that's the plan,” said Limestone.

“We're still waiting for the official authorization for the underground repository,” explained Marble.

“Those Diamond Dogs have lodged another objection. They’re a real nuisance,” said Igneous.

I ran all this new information on earth pony ingenuity through my mind. This was clearly a major industrial enterprise. “What's a nuclear power plant?” I asked.

Pinkie pulled her cream covered head out of the trifle bowl and gave me an enthusiastic smile. “Ooo, ooo, can I do my Pinkie Pie Introductory Nuclear Physics Demo?”

“Yes please,” I said. I noticed Cloudy looked worried.

“Yay!” cried Pinkie, sitting down in the middle of the table in front of a huge plate of rock cakes. Her mother put a hoof in front of her eyes. “You know about sub-atomic particles?”

“Yes,” I replied. “Protons, neutrons and electrons. Protons and neutrons stick together to form a small dense nucleus, surrounded by a cloud of electrons.”

“Which is your favourite? Mine's the neutron.”

I hadn't contemplated this question before. After a brief pause I replied cautiously, “I like electrons.”

Pinkie picked up a rock cake and held it in front of my face. “Now this is a uranium-235 atom. It's a really big fat atom with two hundred and thirty-five protons and neutrons all packed into a tiny nucleus. Sometimes uranium-235 atoms get fed up with being so big and fat, so what do they do?” - she paused briefly, for dramatic effect - “Nuclear fission! They fission in two!” She tore the rock cake into two pieces sending a shower of crumbs into my face.

“Pinkie!” said Igneous Rock.

“And when they fission in two,” said Pinkie, ignoring her father, “they send lots of neutrons flying out.” She picked out some raisins from the crumbled cake mixture in her hooves and flicked them in all directions. One hit me in the eye. “Then some of the neutrons will hit other uranium atoms, making them fission apart, and make more neutrons, which hit more atoms...”

To illustrate this, Pinkie juggled a dozen rock cakes in the air. By some improbable hoof work, she caused one to break in two, sending raisins flying out; these then hit other cakes, which broke apart in a further raisin explosion, thus depicting a chain reaction. In a few seconds the pile of cakes had been reduced to crumbs. I felt a little annoyed. Those cakes had been very good. I had been hoping to have a second one.

“In a nuclear power plant,” said Marble, “we sustain such a chain reaction in a nuclear reactor, and use the heat it produces to make hot water and generate electricity.”

“Question time,” said Pinkie. “What happens if we pack lots and lots of enriched uranium into a really small space?”

“Err...” I said, and then paused, unsure how to reply. I was trying to translate Pinkie's description into terminology I could follow and fit it around what I already knew. Heavy radioactive atoms such as uranium could decay as she described, and if there was enough present then you would get a chain reaction...

“You're slow Twilight,” said Pinkie, “Pumpkin and Pound Cake got it straight away. There would be a great big gianormous explosion!” She threw her forelegs in the air, sending a plate of éclairs flying into the wall.

The consequences of what Pinkie had said became clear in my mind. Earth ponies had a way of producing a nuclear reaction without any magic. All they need was a certain amount of enriched uranium – with a higher fraction of the unstable uranium-235, relative to the more abundant uranium-238.

“Isn't that all rather dangerous?” I asked nervously.

“Pinkie is getting a bit carried away,” said Limestone. “That would only happen if you had a critical mass of highly enriched uranium or plutonium. That's quite hard to make. We only enrich it by a few percent – enough to run a nuclear reactor. We're making nuclear fuel, not bombs.” The rest of her family nodded seriously.

This did not entirely reassure me. “But what if some villain broke in here and stole some material?” I said. “Wasn't Trixie working here at one point? If she'd had some plutonium when she was wearing the alicorn amulet, she could have done something terrible!”

At this comment, to my surprise, Pinkie's sisters both burst out laughing and had to support each other with their hooves. Igneous looked a bit embarrassed.

“Sorry,” said Limestone after she had recovered from her laughing fit, “that's just such a hilarious idea. Trixie making a nuclear bomb.”

“It's not that funny,” I said in what I hoped was a regal authoritative voice.

“Trixie bluffed her way into a job here,” said Marble. “She talked Dad into hiring her with lots of wild claims about what she could do. It quickly became clear that she didn't know anything about rocks. She thought yellowcake was a custard tart.”

“In the end we just paid her off with a bag of gold and told her to beat it,” said Limestone.

So a Great and Nuclear Armed Trixie wasn't a credible security threat. That was good to know, but I wasn't entirely reassured. There were more intelligent villains in Equestria. This probably wasn't Discord's style, but still, if rock farmers were making such potentially dangerous material, they really should be more careful.

Pinkie slid off the table, shook the cake crumbs out of her mane, and hugged her parents and sisters. “It's been totally super fun to see you again. Thanks for the awesome party, but I need to get back to Sugarcube corner. Can you get me four cans of uranium fuel for the Ponyville PWR?”

Limestone left the room and came back carrying a tray of tin cans, each labelled with a bright yellow and black radioactive trefoil symbol. Pinkie took these and placed them in her saddlebag. “And can I take some cakes for the twins? And some cookies? And you know how much Gummy loves your toffee...”

As Pinkie filled up her saddle bags with as many cakes and sweets as she could lay her hooves on, Limestone left the kitchen again and returned with a mouthful of paper forms. She dropped these on the table in front of her sister.

“Paperwork Pinkie, fill these out before you leave.”

“Aw, do I have to? It’s boring.”

“You know the regulators are very strict about this stuff. We need to keep detailed records.”

Pinkie picked up a pink crayon in her mouth and started scribbling on the form. Out of curiosity, I looked over her shoulder at the forms headed with the logo of the Equestrian Environmental Agency. At the top Pinkie had written: 'Dr P. D. Pie BSc MSc PE PhD (Nuclear Engineering, Maresachusetts Institute of Technology)'.

“You're a professional engineer?” I said.

“Yeah, you think they'd let me run a nuclear reactor without proper qualifications?” replied Dr Pinkie.

“You run a nuclear reactor?”

“Why else would I need uranium fuel?”

“But where?”

“Sugarcube corner of course. In the basement.”

“In the basement?”

“It wouldn't fit in my bedroom.”

“You run a nuclear reactor in the basement of Sugarcube corner?” I asked slowly, trying to piece together these new facts and redraw my picture of my sugar loving friend and her Ponyville home.

“Yeah. A small one. And you should totally come and see it. It's really cute.”