Twilight's first twenty minutes or so of having a penis were quite unlike what she had expected. Going off of her previous experiences with Applejack, Rainbow Dash, and Pinkie Pie, she had been quite sure that she would have been on the receiving end of some sort of sexual advance within five minutes of sprouting the extra anatomical feature. Or at the very least, that both herself and Celestia would be actively resisting the temptation to make practical use of it. But unbelievably, Celestia maintained her perfect professionalism.

So far she hadn't even touched the penis yet. She had merely scribbled pages in her notebook, all while mumbling to herself. Twilight wasn't sure whether or mentor was impressed by the organ or whether it didn't phase her at all. The penis itself didn't seem particularly interesting to Twilight in and of itself. It was, as far as she could tell, of average size and dimensions. It had a squishy-looking foreskin around it, loose enough that if you looked at it straight on you'd be able to see the tip. The only attribute it possessed was that her scrotum dangled quite a bit; despite the fact that her flaccid cock hung almost straight down, her balls hung just a bit lower.

"This is quite interesting," Celestia said to herself. Her eyes darted between Twilight's actual penis, and the sketches of other penises that filled Twilight's notebooks.

"What's interesting?" Twilight asked. "Is there some thing weird about my-" She paused. It sounded incredibly strange to be talking about a penis as if it were a regular part of her. "the penis?"

"Not the penis itself," Celestia answered, rather academically. "I was simply comparing you with the data you've gathered of your other... test subjects. I wanted to see if there was any correlation between their original body type and the characteristics of their penises. It doesn't appear that there is, though. As far as I can tell, the size and shape of the penis and testicles seems to be entirely unrelated to what they looked like ahead of time. Given how closely related to hormones these spells are, I thought that their might be a relationship to chest or hip size."

"O-of course."

"I believe I've obtained about as much information as I can from mere observation," Celestia said. "Do you mind if I...?" She held up her hands, the meaning of her question was clear.

"I... go ahead."

"Let me know if any thing hurts, or if you want me to stop," Celestia cracked her knuckles. "If I truly wanted to be rigorous I ought to be wearing gloves, but I don't think it'll be necessary for this. Now then..."

Twilight clenched her fists and balled up her toes as Celestia's fingers touched her penis, squeezing it gently between her thumb and forefinger at the base, where the shaft of her cock met her body. Her whole body trembled as Celestia slowly worked her way down the length of her cock, feeling it between her fingers. As she reached the tip, Celestia slowly lifted Twilight's cock, draping it over her index finger. She raised her other hand and gently cupped Twilight's scrotum and fondled her testicles gently.

"So smooth," Celestia said. "Do you remember if your friends' penises were similar in nature?"

"I-I think so," Twilight mumbled, trying not to think about how inexplicably nice it felt to have her cock touched. The last thing she needed now was to get an erection. Despite never having had one before, she could feel it coming on.

"I see." Celestia gently pressed her finger against the patch of skin between Twilight's dick and nutsack. "How does that feel?"

"It's... uh..." Twilight searched for something to say that wouldn't make her sound like an enormous pervert. "Not unpleasant."

"Good. Now to examine the foreskin." Celestia turned her attention to the end of Twilight's cock, drawing the circle of skin away from the head. Twilight could feel the shaft of her cock beginning to stiffen. Even if Celestia's fingers weren't supporting it, it would continue to stick out.

Twilight inhaled sharply as Celestia softly squeezed the head of her cock. She found herself wishing that Celestia would do a bit more. Nothing much, just that it would feel really good if she worked the shaft a bit. Or if she licked it. That would feel spectacular. The thought alone hardened her cock a little bit more. She tried not to think about that, however much she wanted to. A boner would only make things worse. Or at least, a total boner would. She already had a pretty solid semi-chub.

"Ah, it seems that you're getting a bit... stimulated," Celestia noted. She released Twilight's penis, which now pointed slightly upward under its own power. "We don't need to continue."

"N-no, it's fine," Twilight mumbled. Despite the uncomfortableness of the situation, she found herself wanting Celestia's hands on her dick again. Now she just needed a reason. "I mean... you need to record it's characteristics when it's... up, right?"

"I'll admit I was rather curious about how your anatomy would react to sexual arousal, but assumed that it would be too straining."

"No! No problem at all!" Twilight babbled as she tried not to think about the fact that her boner was getting more pronounced with each passing second. "It's all in the interest of completeness, right?"

"I suppose so," Celestia said resignedly. "Shall I continue, then?"

Twilight nearly said "Please do," but managed to hold her tongue and say "Alright," instead.

Celestia resumed her tactile exploration, once again starting at the base of the cock and making her way slowly along the shaft. Twilight closed her eyes and was immediately bombarded by her imagination. The scene was mostly the same, only Celestia had a someone more... lustful expression, as if she were taunting Twilight with the bare minimum of contact. In Twilight's mind, Celestia voraciously licked her lips. Twilight trembled as Celestia's fingers neared the tip of her cock. Celestia paused.

Then Twilight's mind kicked into overdrive. The Celestia in her mind leaned back, and released a single button on her shirt. The clothing lost its battle against her voluptuous chest, and her breasts burst free, bouncing magnificently. The imaginary Celestia grinned, and gently pinched the head of Twilight's cock.

Any control that Twilight had over her own body vanished. A sudden eruption of glorious pleasure flooded her hips, which seemed to be rocking back and forth on their own. Her knees felt like like they had disappeared entirely, replaced by jello. Her breath degenerated into ragged gasps. Her shaking hands guided themselves to her breasts and she feverishly groped at her own chest. Her eyes sprang open just in time to see her brand new penis blast spurt after spurt of cum straight into Celestia's face. The tingling in her crotch overwhelmed her as she grabbed the table to steady herself as the tension of her orgasm subsided.

Then her mind snapped back to reality, and she realized that she had just spooged all over her mentor, who also happened to be the ruler of the entire nation, and more importantly was responsible for her grades.

"I-I'm so sorry!" She gasped. Her fingers grasped at the air, not sure what to do. In the momentary silence, a final blob of cum dribbled out of her penis and landed on the floor at Celestia's feet with an audible plop. Twilight's aim, while fully accidental, was laudable. Nearly every shot had landed squarely on her teacher's face. There was a streak of semen across her forehead, a splash on each cheek; it was even on her lips. And now it was starting to dribble down and land in large drops on Celestia's shirt.

Celestia cleared her throat. "It appears that you are a bit... overstimulated." Twilight couldn't tell whether Celestia was shocked, angry, surprised... there was no discernible emotion. Twilight couldn't imagine anyone else who could possibly so blase about having gotten a load of cum unexpectedly unloaded into their face.

"I, uh, I just thought that you might... want a sample. Y-you know. For analysis."

"Analysis, really?" Celestia frowned, and the cum trail on her forehead oozed a little. "If you had truly intended to provide me with a sample, wouldn't you have attempted to contain your ejaculate in some fashion? Instead of allowing yourself to ejaculate on me?"

"..." Twilight opened her mouth, but no words came out. "Let me get you a tissue."

"Yes, that would be helpful." She watched as Twilight scrambled around the library, looking for anything that could help her clean up. "Twilight, will you be honest with me?"

"O-of course!"

"Why did you cum in my face?"

"Uh..."

"If you were really intending on a sample, you would have put on a condom, or at least had some sort of container to aim for." She took a tissue from Twilight's hands and began to wipe off her face. "It might be that you were simply unprepared for the stimulation, though in that case you could have warned me. Or else it means that you did so intentionally. I would rather like to know whether this really was an accident on your part."

"I..." Twilight's boner had withered, and now her dick hung limply, dripping the last bits of cum. "It... it was a little of both, honestly."

"Both?"

"I didn't plan to... do... that." Twilight chewed on her own lip. "It's just... it felt really good, and I just sort of... let it happen. I'm so sorry! I'll try my best not to do that again! Even if it did feel really really amazing!"

"I can tell..." Celestia muttered. She nodded at Twilight's legs. Aside from the cum, Twilight's thighs were slick and dripping with additional secretions, courtesy of her female genitalia.

"I'm sorry!" Twilight wailed again. "I know how horrible and awkward this must feel! This whole thing was a stupid idea. I should've been better about my studies so I had something better to present."

"Twilight, I get it!" Celestia plucked another tissue out of the box. "I know exactly how embarrassing this situation is. After all, I'm the one who was in the line of fire, so to speak." Then, unexpectedly, she grinned. "At least you got an orgasm out of it. How do you think I must feel?"

"I... uh... I'm... sorry?" Twilight mumbled, trying to think if she had just heard what she thought she had heard. Had Celestia just made a sex joke? Aside from a few biology lessons, she had never even heard Celestia talk about that subject at all. And even then it was strictly in an academic context. Twilight had never heard of Celestia ever having a boyfriend or anything. She had always seemed the "married to her work" type. Luna... one could imagine her having a roll in the hay with one of the royal guards or maybe summon a royal gigolo. Celestia, not so much.

"What, did you think that I don't have any sense of humor?" Celestia smiled. "Don't get me wrong, I'm not exactly happy that you... well, let's not mince words. You jizzed on my face. But I am I right in understanding that you just got caught up in the moment, and didn't orchestrate this situation so that you'd be able to give me a facial?"

"R-right..." Twilight mumbled, still trying to wrap her head around the fact that Celestia had actually used the word "jizzed". "It's just... I completely underestimated how it would feel. I didn't realize that I would... you know... so easily." She mumbled to herself. "I kinda understand why everyone keeps trying to get me to cast that spell on them."

"What was that?" Celestia asked.

"Nothing!"

"No, I heard you mention that your friends want you to cast this spell on them. Is it really that good?"

"I... Well, Rainbow Dash and Pinkie Pie really seem to enjoy it. They're in here all the time. Applejack is a bit more restrained, but occasionally they'll rope her into their... I don't even know what to call it. Once they talk her into it, she seems to enjoy herself, I guess."

"What do you think?" Celestia crumpled up the final tissue and tossed it into the trash. "Is it really that pleasurable?"

Twilight thought for a moment, then nodded. "Honestly, yes. It felt amazing. I'm not sure how to describe it."

"I see." Celestia stroked her chin and stared contemplatively at Twilight's twig and berries. "I suppose you haven't been able to get useful descriptions from your other test subjects?"

"No." Twilight shook her head. "Applejack doesn't like talking about it, and Rainbow Dash only describes it, and I quote, as 'fucking awesome'. I haven't dared to ask Pinkie yet."

"And I suppose that your other friends are reluctant to help you out?"

"Can you imagine Rarity or Fluttershy willingly growing a penis?"

"Well, if it feels half as good as you seem to think that it is..." Celestia cleared her throat. "A girl could get curious."

"I suppose that it could- wait a second." Twilight's eyes bugged out a bit as she caught the undercurrent of interest in Celestia's voice. "Are you... what I think you're...?"

Celestia shifted herself nervously and for once averted her eyes from Twilight. "I am merely saying that you are in need of test subjects for a spell that seems to be intensely pleasurable, and it would be... unfair for me to ask you to undergo the experience if I was not willing to do so myself."

"So... you want a dick, then?" Twilight couldn't believe that she was even asking the question.

"I... would not put it quite like that," Celestia said softly. Her cheeks blushed slightly. "I simply want to know firsthand exactly what this spell involves."

A flood of questions poured into Twilight's mind. A few were practical, but most were decidedly puerile theorizing about exactly how directly she would be involved in Celestia's experience.

"Well?"

Twilight snapped out of her thoughts, thankfully before she started getting another boner. "Huh?"

"Are you going to cast the spell or not?"

Twilight took a deep breath. "I guess I am. Are you sure-"

"If you're going to do it, just do it before I have second thoughts."

"Alright," Twilight said as she started summoning up the necessary magic. "Here we go!"

Author's Note

If anybody wants to see the dick I had in mind when I wrote this, go to gelbooru and look up the image with the id 686958. I'd link it directly, but that's against the rules.