It finally happened. It was the perfect moment. Their parents were out, they were alone in the house, and Starshine and Blue Monday were a filly, and a colt, respectively. They didn’t know what they were getting into, only that they were being naughty and alone in the house for the first time. “Ooh, look what I found!” Starshine said triumphantly, wiggling back from going under their older sister’s bed. She slid it toward Blue Monday, saying, “It’s that book that mom got for sis, that she said we weren’t allowed to read it!”

“How to Sex 101,” he read, looking down at the title. “Why do you think we weren’t supposed to?”

“I dunno,” she said pensively, then asked, “You wanna read it?”

With a gap-toothed grin he declared, “You bet I do! Let’s see... oh it’s got a lot of words.”

“It says that sex is a wonderful act of aff..eeshion between a loving...”

“That’s affection,” Blue Monday said looking over the blue filly’s shoulder.

“Well *you* read it then!” she declared in a huff.

“It’s a wonderful act of affection between a loving... cowple,” he said slowly, looking at the words in puzzlement.

“Oh that one’s couple,” Starshine said cautiously.

“When a colt and a filly love each other very much,” he continued without more than a sideways glance of a comment, “They have a special hug, called mounting, where the stallion puts his...”

“Wait, stallion?” Starshine asked, “I thought they were talking about a colt and a filly?”

“It says he puts his *penis* inside her!” the colt declared in shock.

“W-what? I mean, do penises go inside ponies?” she asked in confusion.

“I don’t know, it’s just what it says!” he said pointing a hoof at the text.

“Well you have a penis, right?” she said, blushing irritably.

“Uhm... yeah?” he replied uncertainly.

“Well I don’t have a penis,” she replied, blushing harder, “So you know more about it. I don’t know anything about... penises.”

“Well—you—you *don’t* have a penis, do you...” he realized in shock.

“M-mama says it’s a fillyhood, that’s all she ever called it,” Starshine said self-consciously, “You really wanna put your penis *inside* people?”

“No, that’s just what the book said,” he replied, “All I do with my penis is pee. It just comes out, and then the pee comes out of it.”

A pause, and she said, “I’ve never seen a boy peeing before.”

“Woah, we could... we could both go into the bathroom and pee!” Monday declared excitedly, “We won’t even get in trouble! You wanna see how I pee?”

“You wanna see how I pee?” she replied testily.

“Yeah!” he said happily, “Let’s go. I wanna show you my *penis.*“

“Fine, but I don’t have anything to show you, just a uh... butt,” she said.

In the bathroom, he stood over the toilet, and his penis slid from his sheath. “Wow, it’s... getting bigger,” she said in fascination, staring beneath the colt.

“Yeah it... comes out, I dunno. So the pee can come out of it. I–I’m gonna pee,” he said indulgently, “So um... umnh.” And a stream of urine started to shoot out of the flat tip of his penis, arcing down into the toilet.

“Oh, so that’s why you stand forward,” Starshine said, watching the colt peeing, “Your penis just... pees it in there just perfectly.”

“Yeah uh... it’s a little weird,” he said, blushing, “You don’t mind, do you? I can’t stop until it’s done, so I just have to s-stand here.”

“Oh, it’s the same for me, sort of,” she said vaguely, watching the stream of urine strengthen then trickle off. 

“Okay, I’m done,” he said, as his penis bobbed strongly in the air, then slapped up against his belly. “O-oh sorry, I just do that to get out the rest of the pee,” he said shyly.

“No, it’s fine,” she replied, “Cool, and your... penis just goes back into you. I wish I had a penis!”

“Well what *do* you have?” he replied, “I haven’t seen a filly peeing before either!”

“It’s not anything special,” she grumbled, as he moved away from the toilet, “I just have to face the other way, and it just comes out. I’ll just um...”

“Oh, I see you have to... stand facing the door,” he said, “Okay, I see under your tail... woah, you do have a fillyhood under there!”

“What do you mean?” she replied anxiously.

“It’s like a slit,” he said, “Or a pocket or something. You just um...” She squeaked as his hoof poked her muff curiously. “Oh, sorry!” he declared, pulling it back, “It’s okay if I touch it, right?”

“Well we’re not supposed to let anypony touch us down there,” she told him, “So yeah, touch it! I won’t tell on you. And also I can touch your...” an excitement rose in her, “...penis, too.”

“Oh yeah, of course,” he replied confidently. Then she squeaked again as his hoof prodded her mysterious fillyhood. “It’s really soft...” he said engrossedly, “You can kind of... pull the slit open...” on his haunches, he touched her with both forehooves, making her shiver at the feeling of her labia being spread wide.

“Woah!” Monday exclaimed, “You’re really pink in there!”

“I am?” she asked uncertainly.

“It’s not bad,” he insisted, “There’s just no fur inside, and it’s kind of... gooey.”

“Gooey?!” she squeaked indignantly.

“It’s not bad, just kind of wet in there!” he said, “And it... smells *really* good.”

“You’re *smelling* me??” she declared in outrage.

“No, really! It smells good!” he said, and her hind legs stiffened as the side of his hoof slid along her inner labia. He trotted around on three legs then, saying, “See? It smells good!”

Cautiously sniffing, Starshine said, “Wow, I didn’t know, it’s... kind of spicy? Or... lemony?”

“I dunno, but it’s...” he licked his foreleg experimentally, “Really nice smelling. It is kinda lemony. Do you think I smell good down there too?”

“Only one way to find out!” she declared.

He turned his rear to her, raising his tail, and she nosed under there, finding and definitely avoiding his puckered little anus. “Oh there are... things here,” she said uncertainly, nosing at his balls.

“Oh, those are my balls. Um... you don’t have any balls back there, so it’s just a colt thing,” he said.

“You smell... *amazing*“ Starshine declared, the humid spice emanating from his little ballsack going straight to her groin as she smelled it. “Not lemony as much, but really spicy. Wow, that’s cool!”

Her tongue probed his soft little balls, but the skin was only salty, not especially tasting like anything.

“H-hey, are you *licking* me?” Monday declared in surprised outrage.

“Sorry, I just thought... since it smelled good... sorry!” she replied, backing up from his groin. 

“No, no it’s okay, it didn’t hurt, I just didn’t think you could lick that,” he replied appeasingly, turning towards her, “So I can lick *your* ...place?”

“Well, I... think you can,” she said testily, turning her young rear his way, lifting her tail, “It’s only fair since I licked yours. I dunno even what I was thinking, that it’d taste good, I guess.”

He nosed under there, and once again his forehooves spread her labia, but this time his *tongue* it just.... slid along her in there, making her groan in surprised pleasure. “Are you okay?” he asked worriedly.

“No I’m fine, it just surprised me,” she said uncertainly, “Your tongue was really licking um... sliding? It didn’t feel bad, just weird.”

He seemed like he was going to address that, but then Starshine started breathing harder, as her brother slid his tongue up and down her slit, like she was an ice cream cone. It was tingling strangely down there, and getting really intense. She wanted to say something, but she was afraid if she did, he’d stop! Then something *surged* in her and she had to stay something.

“Monday wait, I–I really need to pee!” she declared in rising panic.

“What?” he asked, and the release of the pressure of his tongue made everything back there flex trying to hold it in, and a little squirt of urine just burst out of her. 

“Hold on, I just—it just started really needing to,” she stammered shamefully, hastily backing over the toilet. Releasing her bladder, her urine came out with a heady rush of relief, a strong stream pouring out from her labia and descending into the toilet. “Okay, I’m... peeing now,” she admitted, flat-eared.

“Oh, oh I wanna see, oh—” he said, hurrying back there, then stopping. “Huh, it’s just coming out of your slit,” he said curiously.

“I guess that’s where my penis would be, if I had one,” she said, blushing as her bladder compressed. “It’s really weird to just pee in front of you,” she concluded uncertainly.

“Maybe it has to do with that sex thing,” he said, “They were talking about my penis. Maybe they talk about your um...”

“Fillyhood,” she said, finishing up back there.

“Woah, it’s moving!” he declared, “Like, scrunching up and stuff!”

“Oh, I do that to g-get the last of the pee out,” she said, ceasing her unconscious winking, “Just like how you jerk your penis, I guess.”

“Oh, so you wipe?” Monday said, as Starshine reached for the toilet paper.

“Hey, I don’t have a penis to make it easy,” she fussed with a blush.

Her blush deepened as he asked, “Can I wipe you?”

“Sure, go ahead!” she said in false confidence, lifting her tail way up as her brother hoofed some toilet paper, and... then somepony *else* was wiping her. 

It felt... wrong, yet right. He wiped her off good, saying in delight, “Your fillyhood is so soft and cute!”

“Yeah, well, thank you for um... c-cleaning me,” she said, trying to understand what was happening to her with all this contact to her fillyhood.

“It doesn’t even smell like pee anymore,” he said, after dropping the toilet paper in the toilet, and flushing. Then he started licking her again.

“Aah!” she squealed, then frantically said, “No, no it feels good! I didn’t mean to make that sound. You can lick it. Oh oh oh huh... huh... huh..” her words fell to panting, as she stood there, slightly rocking back. She felt like her fillyhood was gonna explode, as his tongue slid around and through her labia, and his own lips pressed against her, and rubbed around. It was so overwhelming, she couldn’t even think. It wasn’t that it felt good, so much as she needed to feel this way.

Then he stopped.

“Wh...why did you... stop?” she asked, looking back his way dizzily.

“My... my penis came out,” he said honestly, turning to show her. “I can’t pee though. I already peed. It... w-won’t go back in.”

“...can I lick it?” she asked.

So he lay on his back there right outside of the bathroom in the hallway, while she crouched before his groin, and licked his penis. As Starshine did, Monday’s penis grew and bloomed in size. “Why’s it getting... so big...?” he asked in  lust-addled confusion.

“It’s your penis, you tell me,” she said cheekily, sliding her tongue up and down it, “I think my licking is making it bigger.”

Then she outright suckled on the tip, and he bucked his hips up with an intense cry, shoving that spicy, smelly penis all the way up into her throat. She gagged at that, because ponies are not horses, pulling back and coughing as his penis left her mouth and slapped against his belly.

“You tried to shove that whole thing inside me!” she accused him once she’d gotten her breath back.

“It just happened!” he protested, “My hips just jerked up and—I didn’t mean to.”

“They just... jerked up?” she asked.

“Yeah, like a... like a reflex,” he said, “Like at the doctor with the hammer.”

“Oh, well... it wasn’t supposed to go inside me then, I guess,” she said thoughtfully.

“No, it *was* supposed to go inside you!” he declared, “That’s what the book was talking about!”

“I almost choked!” she replied accusingly.

“Well—” he blushed, “Maybe we’re... doing it wrong. Let’s look at the book again.”

“Okay,” she said, “But I am not putting that whole thing in my mouth!”

“Maybe it’ll get... little again?” he asked with a sympathetic wince.

With his erection bobbing between his legs, and her own arousal dripping down hers, Starshine and Blue Monday were quite ready for something to happen. Their arousal was cooling though as they pored over the text.

“It goes in her vagina?” Starshine asked in confusion, “What’s a vagina?”

“Search me,” he said with a shrug, “I don’t even know what this intro mission thing is.”

They turned the page, and Starshine’s eyes lit up, as she said, “Oh, there’s a diagram—”

“...”

“I guess that makes sense,” he said incredulously, “So your fillyhood is like a... inside penis?”

“I... I dunno!” she said, staring at the cross section drawing of the stallion mounted on the mare, with his penis clearly hugged up inside a... thing. “I never tried to *put* anything in it.”

“Well, lemme try,” he said, “I’ll just try to put it in a little, just to see if my penis can do it.”

“Yeah, that... that’d be good, I think?” she said uncertainly, “It’s not your tongue, but... do you really think you could get it all the way inside me like that?”

“I’ll just put it in as far as I can,” he said, “Just to see how big you are in there.”

“Seems like as good an idea as any,” she said with a shrug, the filly turning her rear to her young brother, and moving her tail aside to present her labia. “I sure don’t know how *else* to figure out if I have something that goes all deep in there.”

He mounted her, and a blush crept back on her cheeks as she said, “Oh wow, m-my legs are getting stiff just like... just like in the book. It feels *weird* when you grab my flanks. N-not bad though.”

“My penis is just right about...” then he slapped *her* belly with his penis. “Right about in the right place,” he said. 

“Oh, that’s why you can jerk it!” she declared in realization, as he lifted his penis to press into her soft slit, “So you can lift it up to put in my vagina.”

“Wow, you... you feel *really* soft with my penis,” he said distractedly, his shaft slipping up and down her increasingly slippery labia as he jerked it against her.

“You’re—” she said with a solid blush, “You’re practically *licking* me with your penis.”

“I wanna try putting it in though,” he said, working his penis into her labia. Starshine felt her fillyhood quickly stretched around his girth. His penis was young, but so was her fillyhood, so there wasn’t as much wiggle room as one might think. “Oh wow, you got really um... tight all of a sudden!” he said.

“No I didn’t!” she protested, “Your penis is just really stretching me!”

“Does it feel bad?” he asked.

“No it’s... it’s fine,” she said, wiggling her butt testily against the penis poking into it. “Try um... try penetrating me. Just push a LIT—le—!”

His shaft just *sank* in her then, spreading her *wide* open. “It’s working!” she declared in triumphant shock, as the horny colt on her rump humped deeper into her, “I can feel you deeper and deeper! Oh my gosh, I have a *vagina!*“

His hips collided with hers then, and the colt quivered against her, his shaft twitching inside her. “Starshine, you’re *hot* inside!” Monday declared passionately.

“Hot?” she asked in confusion, looking back at the colt.

“It got *really* hot and wet,” he said, as he hugged her, “Like even more than your outside fillyhood. It’s really wet and... and slidy!”

“I-is that good?” she asked anxiously, wondering if she was too hot around that penis that made her feel her vagina for the very first time, quivering around it hungrily.

“Yes, I like it,” he said eagerly, “It felt really good to slide...” she sucked in a breath at the feeling of him sliding out of her, “...in,” he concluded, sliding slickly back into her. “Just like...” he slid out again, “Like this.” he shoved in again.

There was a moment of stillness.

Then he was *sliding* out and *sliding* in, out and in, out and in. “Oh, I *love* the sliding!” she moaned in surprise, her whole slick passage stretching open with every penetration, the achingly stretched walls tingling fiercely as that penis slid against them. “My vagina loves sliding,” she repeated intensely, “Loves your penis... sliding... into it.”

Then he was fucking her hard. In and out with a primal, ancient rhythm that roused the unknown flutters of orgasm in her groin. It didn’t just feel good. She *needed* it. She needed that penis that was filling her again and again. She almost stumbled onto the book, then she shakily closed it and stomped on the cover, rocking back as he fucked his huge penis in and out of her.

“Monday!” she squealed needily, her vagina writhing all around his pistoning shaft, “My vagina keeps... grabbing you!”

“D-do that, please!” Monday insisted, “I feel so good! My whole penis is tingling!”

“My vagina’s tingling too!” she squealed back, “It keeps getting... more and more!”

“Oh Starshine,” he moaned anxiously, “I *need* to put my penis... deep...”

They just kept fucking then. Starshine sat back against his thrusting so she could clumsily slide a foreleg between her legs, pressing up on her belly, in which her brother’s penis was sliding. His penis was sliding all in and out of her tingling vagina, and it wouldn’t go down! Her vagina’s intermittent squeezing stuttered to a halt, as everything... prepared back there. He thrust in and out smoothly as all she could think about was how big his penis got, and where it was right now, squishing up against her deepest places again and again.

She couldn’t stop it from getting bigger every time he thrust into her. Bigger, bigger, bigger, it was like getting electrocuted with pleasure. A tight whine rose in the back of her throat as all her muscles seemed to tense at once. A little flutter then grew and grew and grew and before she could so much as squeak her whole vagina was convulsing in powerful rhythmic clenches, sending waves of pleasure crashing through her whole body.

“Starshine, what’re,” Blue Monday gasped as she helplessly clenched around his shaft, more than her vagina had ever moved before.

“I can’t stop!” she squealed, her words sliding into a high wail of ecstasy.

“Sis I can’t... it’s...” he tried to fuck as her inner convulsions began to fade, but his thrusting was shaky, jittery even. His hooves hugging her tensed more and more, as the filly looked back at him, feeling like she was glowing inside, but concerned as he just looked her way in fear, with a tight whine rising in the back of his throat.

“Oh, it’s... okay, do that!” she said eagerly, if a bit breathlessly, “Make it bigger and bigger!” as his thrusting grew stronger again, she said dizzily, “That happened to me just now, and it’s amazing I couldn’t stop...” then he shoved deep into her, his whole body tensing with an urgent, desperate look in his eyes as it happened to him. “You’re gonna start clenching now,” she said, “I don’t know what thoug—” and then his penis started bucking inside her, splurting semen up into her in deep spurts.

“What are you—!” she squeaked, as every time his penis bucked inside her, a blast of something hot and slimy rushed into her, right where the tip was thrashing within her. “You’re putting something inside me!” she whined, putting her hoof between her legs again as that stuff he was pumping into her started to escape around his penis and slide right out of her.

Every spurt raised a primal urge within her, to fill herself with this stuff. She cried out in desperate relief as he cried out in desperate relief, pushed his big penis into her, and her vagina quivered and clenched again in a pseudo orgasm at the feeling of utter fulfillment, as everything she could have dreamed of happened inside her with those hot splurts.