So,what did you need?” Spike asked, smiling at his guest. He hadn’t seen Rarity in a while and missed their talks.

“Oh, nothing darling.” She insisted, waving a hoof. “Just visiting.”

“Really?” He echoed, cocking a brow.

“Yes, absolutely.” She looked toward Thorax, who was glowering at her from behind his mate. They didn’t get along though Rarity had no clue why. “Good evening, Thorax. I pray you are well today.”

“…” The changeling remained silent, ready to strike if this pony got too close. He couldn’t allow anyone to get between him and his mate.

“Uh… right.” Rarity felt a bit awkward but powered through. She had been in more tense situations before. “Anyway, how is the little one? Fine, I assume?”

“Yeah, he’s napping.” Spike lifted a wing, revealing his little cub to Rarity. He was still very protective so she knew to not to get too close. Friend or no, getting close to a dragons child was suicide.

“As do most foals.” She nodded, thinking of her own little foal. “Little Crescent always takes naps around this time.” They continue to exchange pleasantries until Rarity feels it was the right time to ask what she wanted to. It’d be rude to start out with a request after all. “So, Spike, this might sound strange but… Was Little Luce hatched from an egg? Like a dragon egg?”

“No, he came from a Changeling egg.” He shook his head, thinking back to that time. “A bit different.”

“Really? I had no idea they were different.”

“Yeah, Thorax was confused too when I explained how our species’ eggs are different.” Thorax huffed, a bit embarrassed to have that brought up. Spike chuckled and nuzzled him briefly. Rarity had enough tact to pretend she hadn’t seen such a display. “Like, did you know that changeling eggs are soft and malleable? They’re like little marshmallows! It’s crazy.”

“Really? How do dragon eggs differ?” She asked, mentally taking notes. “Do they not hatch like changeling eggs?”

“I mean, kind of.” He shrugged, not sure how to explain it since he only knew from ancient dragon texts “They won’t hatch unless they are imbued with enough magic or heat.”

“Heat.” She muttered, thinking of her own egg. “I see, so you sit on them?”

“Dragons aren’t birds, Rarity.” He deadpanned, rolling his eyes. It wasn’t the first time someone had come to that conclusion. “We light them.”

“L-light them?” Her eyes snapped open. “Like, on fire?”

“Well, it can’t be just any fire obviously. I mean, it wont harm the egg, since they are made to survive for decades if need be.” He explained, shrugging his massive shoulders. “No, you need to light them with dragon fire.”

“Dragon fire…” Rarity mumbled, frowning to herself as she pondered her predicament. 

“Yeah, I don’t get the science of it but something in dragon fire makes it easier to hatch the egg.” Spike continued on, unaware that the mare was only half listening. “Twi says it’s because dragon fire is magical but hasn’t figured out the whys and hows. Honestly, you’d think over a decade with me around would make her an expert on dragon fire.”

“Yes, one would.” She distantly replied, an idea filling her mind. “I see.”

***** 

Takes place briefly after this: 

When Rarity took a dragon egg as her own, she didn’t expect that it’d be this hard to hatch the damn thing. Like most ponies, she didn’t know much about dragon biology and she couldn’t go and ask Dragon Lord Ember for help. So she went to the next best thing. Her buddy, Spike.

Spike, while having the opportunity to branch out, spent a lot of time learning about his species from other dragons as well as texts enshrined deep in the Canterlot Library. While he hadn’t had the chance to hatch his own eggs, he found great value in learning more about dragons.

Now having learned how eggs hatch, Rarity has two choices. 1: Find a dragon willing to hatch an egg, no questions asked or 2: Find a unicorn strong enough to hatch a dragon egg, no questions asked. Neither is possible and Rarity is forced to make a third choice.