Celestia and Luna were enjoying retirement. They could do what they wanted with their, had no more royal duties and still had the luxuries of guards to protect them, servants looking after them and an excellent healthcare team. However, they had not spent much time together over the years, and Luna had forgotten one crucial detail about her older sister: she does NOT eat spicy food. You would have thought that Luna would not overlook this detail. That fateful day when Celestia first tried spicy food as a filly, no matter how many thousands of years ago it happened, was hard to forget. Yet she somehow let this slip from her mind, and the results of this were very, very ugly.
One night, not long after Princess Twilight Sparkle had taken the golden reins from the Royal Sisters, Luna made dinner for the two of them. She made a wonderful salad and added some extra-spicy chilli sauce of her own concoction. On those long, lonely nights when nopony had a nightmare, Luna would have trouble staying awake. There were only two things that kept her up all night: coffee and a spicy meal. She now used this sauce on her lunch to help her adapt to staying awake in the day and sleeping at night. She loved it so much and she wanted her older sister to try it. She was so confident that Celestia would love it. How very wrong she was.
The first five minutes of dinner were uneventful. Between mouthfuls, the sisters shared small talk about what they had done that day. Luna had been exploring some of the small shops in Ponyville's High Street and visited Rarity's boutique to talk about getting a new outfit that wasn't a dress. Celestia had been to a library. She was not quite as bookish as her successor, but she loved her gothic novels. Luna remembered when Celestia had been a teenager and dressed all in black and used a spell to dye her mane and tail dark blue. She had barely taken her muzzle away from the yellowed pages of the books she read. It had practically dragged across the writing. The sisters shared this memory with a few giggles. If only Luna had remembered about Celestia and how she never ate spicy food.
It was then that Celestia started acting strangely. She started getting very chatty and not much of what she said made sense. Then she couldn't sit still: she kept jumping up and down in her seat. Luna assumed that it wasn't very comfortable, which worried her; Celestia always liked to sit in a comfortable seat, and barely moved to make sure she stayed in her comfy position. When she had been ruler of Equestria, she had kept an invisible cushion on her throne at all times because she said that the throne hurt her buttocks. This all made Luna begin to silently wonder what had happened to her sister and whether she was in any danger.
"Is this as spicy sauce?" asked Celestia.
"Yes," replied Luna, "why?"
"Uhohthisisbad. You four got. Spicy spicy."
Luna remembered then what had happened on that day when Celestia had eaten spicy food for the first - and, up until recently, last - time . This had happened. She knew what was coming.
The laser beam shot past her ear. She felt it rushing past. She turned around and stared at the brand-new hole in the wall.
"Celestia," she said warily, "do you need some water?"
The answer was a sizzling flame leaving her sister's mouth. Celestia's eyes were glowing bright red. This was why Celestia's food was always so bland! How could she have forgotten!
Celestia's magic made another hole in the wall. She was glowing orange all over her white coat. She looked like she would burst into flame at any moment. Luna rushed to the cupboard just as Celestia stomped with her front hooves and the floor cracked. The ex-ruler of Equestria could destroy the land if she didn't have relief from the miniature heatwave in her mouth. Using magic, Luna grabbed a glass and filled it with tap water. Now for the hard part. If this was anything like last time, the fate of Equestria lay in Luna's hooves and this glass of water.
Luna teleported right next to Celestia and tipped the contents down her sister's mouth. Her eyes changed back to their normal colour and she could just say, "More please!" before a flame shot out of her mouth. Luna repeated the procedure three times before Celestia returned to normal. Well, normal but angry.
"You gave me spicy food," she snapped.
"Yes, and halfway through your meltdown, I remembered why not to," countered Luna.
"That was the most horrifying thing I've ever tasted. It felt horrible."
"It killed me watching you react."
"That sounded really sarcastic."
"I would have been laughing if I didn't know that you could have destroyed Equestria while you were upset about the flavour."
"It burned my mouth!"
"Two things. One, it's not an actual burn, it's just a chemical reaction. Two, it's supposed to taste and feel like that. Some ponies enjoy it. It's clear that you don't."
"NEVER do that EVER AGAIN," Celestia warned her sister with the voice of Canterlot.
"I do not need to be told twice. Although, it would be funny to film..."
Luna's voice died under her older sister's glare.
"Okay, I was just kidding!" she pouted.
The next day, Celestia petitioned to her former student Princess Twilight Sparkle to abolish all spices. As Celestia was the only pony to sign it, Twilight kindly refused and explained that she ate spicy food herself from time to time. Luna had learned a valuable lesson in how Celestia liked her food - bland. The alternative was a monster who was stronger than Daybreaker and had the power to destroy the world. Luna wrapped her sauce in several layers of brown tape and put on a sticky label that read, DO NOT GIVE TO CELESTIA! She vowed never to give her sister a spicy meal ever again.