The doorbell rang, snapping Tails out of his focus, and he looked up from his book. The normally annoying interruption was welcome instead for two reasons: the first was that he was getting nowhere in his current avenue of research, and the second was that the visitor at his door was most likely the way he was going to make some progress.

Sitting up in the chair in front of his messy workbench, Tails turned to the monitor at one side of it and turned a dial near the bottom, bringing up the display from the camera at his front door. As he had both expected and hoped, it was Amy who was there, looking directly at the camera. The little green light on the camera told her that he was watching her, and she gave him a smile and a wave – then blew him a kiss. "Hiii, Tails!" she called out, "Sorry I'm late, hope you don't mind!"

Was she late? Tails glanced at the nearest clock, which was precariously perched on top of a toolbox that, in turn, was on a stack of books. He'd lost track of time, but she was only a few minutes late anyway. "Come on in, it's no trouble!" he told Amy, pressing a button on some controls at the edge of the workbench to unlock the door for her remotely.

"I'll see you soon!" Amy called out to him cheerfully before opening the door and disappearing out of the camera's line of sight.

Pushing back his chair, Tails got up and stretched his arms over his head. The two-tailed fox was only wearing his boots, his tool belt, and the goggles that were currently pushed up to his forehead. He rarely had visitors, so why bother with anything more? And his current visitor wasn't going to mind his state of undress either, doubly so considering what she was helping with...

It was a little out of his usual realm of study, but Tails had learned about the existence of a rare, third gender of Mobians. Not only that, but it had turned out that Amy was one of these so-called 'dickgirls' he'd only just recently learned of, and she'd agreed to his request to let him study her. The only other one he knew of was Sticks, who was going to be his second choice if Amy had declined. He'd been trying to do additional research, but there was very little written about them other than the fact that they existed. But where others might see only frustration, Tails saw an opportunity: he would be the first to write about them and publish a paper! Or some articles, at least. Either way, he could do the research that seemingly no-one else had.

Lowering his arms, Tails looked around. His workshop was a mess, like always, but it was an organized mess insofar as he knew where everything in it was. The screws were in a box at the far end of the workbench, the rubber tubing was in the box holding up the pile of manuals, his wrenches were in the bag next to the Sea Fox, and so on. Sure, he'd last used that submarine ages ago and refused to take apart just in case he ever needed it again, but he just knew that the day he dismantled it would be the day Sonic would come and tell him about some underwater base they'd have to-

Amy opened the door to his workshop and poked her head inside, interrupting his inner thoughts. "One sec," Tails told her, quickly shoving some of the less wobbly stacks of stuff to the sides to clear some space. "There, all set! Come on in."

Amy stepped in, wearing her knee-length red dress and matching red hairband. "Are you ready?" she asked him, sounding a bit excited. "This is going to be so much fun!" she added, flashing him a big grin, her eyes glancing down to his bare loins where his balls hung just under his white-furred sheath.

"I'm definitely excited," Tails replied, not quite noticing the deviousness of her expression nor where she was looking, too lost in his own thoughts. "A whole area of study that no-one's bothered with! I'll be famous!"

"You're already famous," Amy pointed out, "Mister Hero."

"...okay, well, I'll be famous in academia!" Tails conceded. Reaching for his desk, he grabbed the clipboard from its spot between the Tornado's maintenance log and a robotics magazine. "Anyway, I'm going to ask a few questions to start."

Amy tilted her head, holding her hands behind her back. "What kind of questions?"

"Well, for starters..." Tails looked down at the clipboard. "How long have you been a dickgirl?"

"Uh, always?" Amy replied with a shrug. "I was born like this."

Tails nodded, writing her answer down, then looked thoughtful. "So you and Sonic...?"

"He usually bottoms," Amy answered nonchalantly, though she couldn't quite keep a slight playful tone out of her voice.

Tails elected to leave that part off of the answer he wrote down, but it explained a lot. "You hide it pretty well."

"Well, yeah, that's why I wear long dresses. I mean, look at this." Amy reached down and tugged the hem of her dress up her thighs, revealing most of her flaccid dick to him, as well as the bottom of her heavy ballsack, both covered with the same pale pinkish skin as her belly. "I'm not going to hide this under a miniskirt, am I?"

Tails couldn't help but stare, feeling his cock stirring within its sheath. "You're right," he replied, forcing his gaze back down to the clipboard to write on it. "Has it always been that big?"

"Since I became an adult, yeah," she answered, keeping the front of her dress held up so she could enjoy seeing his flustered look every time he glanced over at her girth. With a teasing look, Amy lowered her voice and offered, "Did you want to measure it?"

"M-Maybe later," Tails replied. "Though, actually, I should have you give me a sample so I can let the analyzer run on it while we talk."

"A sample?" Amy inquired playfully, cupping her balls in her left hand and slowly rubbing her dick with her right.

"A urine sample," Tails corrected her, though his eyes were drawn to the little show she was putting on, his own shaft stiffening gradually and peeking out from his sheath. "I was reading a book that mentioned there's a lot you can tell from a urine sample, it contains-"

"I'll need a cup," Amy cut in pointedly.

"Right!" Tails agreed, and turned around. Where had he put it? He knew where everything in his workshop was... normally. But he'd only gotten the sample cups yesterday and had put them... somewhere.

Setting the clipboard down on the workbench, Tails sifted through the stuff on his bench. "Anyway, as I was saying," he began, before continuing to ramble about hormones and spectrographs and chemical compounds, none of which Amy paid any attention to at all. Her eyes were instead locked on Tails's rump and the way it swished back and forth as he moved, both of his tails swishing as he nerded out about chemistry. Or was it biology? Whatever. Amy wasn't listening, and instead was idly stroking herself as her eyes roamed over the fox's ass, admiring his lithe form, feeling her length rise to full hardness.

When he bent over at the waist to peek under his desk and rummage in the piles of things there, his tails lifted up by themselves, presenting Amy with an irresistible view of his ass and hole. Suitably enticed, Amy stepped up behind him and interrupted his monologue. "Actually, Tails, I have a better idea," she told him, putting both hands on his lower back to hold him in his bent-over position. "Can you hold still for a sec?"

"Huh?" the twin-tailed fox called out from under the desk. "Oh, uh... sure. What've you got in mind?"

Her answer was very direct: she pushed the head of her cock up against the ring under his tail and began easing the bulbous tip into his rear. He gave a surprisingly effeminate moan and his tails flicked higher, and she had to tighten her grip on him to stop him wiggling free. "A-Amy, not now! That test isn't until later!"

She raised an eyebrow curiously, but continued to penetrate, sliding just a couple of inches of her dick into his ass. "Hush, and keep still, or you'll spill it," she commanded, and he obediently ceased his movements. "That's good. Now don't move..."

"O-okay," Tails replied, and Amy took a deep breath. Exhaling slowly, she relaxed and her cock swelled out on the underside, a heavy stream of her pee gushing into Tails's ass and hosing down his inner walls.

He let out another moan and squirmed a bit, only for Amy to grip his hips more tightly. "I said, don't move," she reminded him, then gave a soft huff as he kept still and let her savor the feeling of relief inside of him once again as she emptied her bladder into the fox's anal tunnels. Tails let out a moan as well as he felt the liquid warmth pouring into him, and with Amy's dick plugging his hole it had nowhere to go but deeper into his guts, spreading the wet warmth within him.

Tails offered no other words of protest, and Amy smirked to herself as she held the position and emptied her bladder. The tightness of his ass around the front half of her cock made the sensation of pissing all the more pleasurable, and she idly rubbed her hands over the curve of his ass as she continued to relieve herself. He couldn't stop himself from moaning out too, his cock throbbing between his legs as he kept bent over, both tails flicking as Amy kept filling him with urine, one hand rubbing over his belly as the warm feeling from her fluid kept going deeper and deeper.

When she was done, Amy clenched her muscles to get every last drop she had out of her and into the fox mechanic, patting him on the rump again before pulling her shaft back out. He started to move but she grabbed onto his hips once more and held him in place. "Ah, ah, ah, stay put. You don't want to spill the sample now, do you?"

"But I need the sample to..." Tails began.

"Part of what you'll need to research is the effect of dickgirl fluids on non-dickgirls," Amy cut in firmly as she looked around the mess on his desk for something to plug him with. "So, I would recommend that you hold it in for... let's say a couple of hours, at least. Make sure to write down any effects it has on you. You know how to do that part of the research, right?"

"Yeah, of course!" Tails replied proudly.

"Good," Amy said with a nod, and finally spotted something usable: a rack of test tubes. Plucking an empty one free, she brought the rounded bottom of it to Tails's ass and pushed it in with ease, considering it was narrower than her cock. "Remember, keep that in for a couple of hours, okay? And I'll come back in a couple of days to do this again. You can come out now."

Obediently, Tails shuffled back out from under the desk, carefully moving to keep the test tube inside of his rump. "Okay, Amy," he agreed obediently, nodding to her. "I, uhm, I think that's it for today, then. We can do the other tests another time."

"Sure thing, Tails," she said teasingly, leaning in and giving him a smooch on the cheek. "See you around!" she added as she turned, leaving him blushing and erect. She had no doubt he'd be tending to that the moment she left, so she wiggled her ass at him on the way out before closing the door behind her.

--------------------

Humming to herself as she strolled up to the door to Tails's house, Amy reached out and pushed the button for the doorbell. It had been a couple of days since her last visit, and she was eager to see him again. It took a few seconds before the screen by the door turned on, and she saw Tails's face, his goggles up on his forehead again.

"Hey, Amy!" he greeted her, breathing heavily, only slightly out of breath.

"Hi, Tails!" she answered, turning and waving to the camera to her right.

"I've been waiting for you! Come on in!"

The screen went dark and she heard the door lock click, so she pulled it open and headed inside, stepping around a cardboard box of spare parts left by the front door. She knew the way to his workshop at the back and made her way there, ignoring the other miniature robots that Tails had built to help around the house as they either zipped from room to room or busily worked on whatever task they were given. Arriving at the workshop door, she pushed it open and strode inside.

While some parts, gadgets and tools had moved around, the biggest and most noticeable difference was Tails himself. He spun around in his chair as Amy walked in, revealing that he was casually masturbating, stroking at the canid cock jutting from his sheath with little concern for Amy being able to see him. She couldn't help but smirk at the sight-half closing her eyes and asking, "Been having fun?" in a teasing tone.

"Yeah, I don't know why, but this is the third time today already!" Tails explained, gesturing to a trashcan overflowing with tissues with his left hand while continuing to stroke his cock. Then, half-turning in his chair to face the desk again partially, he picked up the pen that was lying on top of the clipboard. "Is it like this for you too? Do you get really horny sometimes?"

Amy gave a little chuckle as she maneuvered around the piles of stuff in the workshop to get to him. "Yeah, I was almost constantly horny for a while. I learned how to keep it in check eventually."

Tails scribbled messily with his left hand while he kept fapping with his right, nodding to himself. "How do you do it?"

"I slip away and jerk off regularly," she told him nonchalantly, holding up her right hand and making a back-and-forth stroking motion with it. "Even we dickgirls still have a refractory period. And the more you try to resist it, the stronger your urges get. So it's best to just give in and bust a nut whenever you start feeling it."

"Y-yeah, I noticed," Tails agreed as he wrote another line. "I tried to hold off after you left last time, because I wanted to get more research done, but I just got hornier and hornier."

"Yep, that'd do it," Amy sympathized. "Anyway, I brought you a present!" she declared, reaching into the pocket on her skirt.

"A present?" Tails asked, putting down the pen and turning around to face her. Now that she was up close, she could see that Tails's body had changed, if only slightly. His chest was just a tiny bit rounder up-front, with two little pink peaks poking out from his white fur. And his hips and ass were just a bit more filled out, almost imperceptibly so, but she remembered how he'd looked sitting in that chair on her previous visit and could tell the difference.

"Yep, right here," Amy told him, taking her hand back out and holding up a shiny new butt plug. "This'll help hold in the second dose I'm going to give you. I even got a custom design just for you!" she added, turning it to show him the cap, which had a stenciled outline of her head drawn on it. If anyone saw it, they'd know who Tails had been having fun with...

Tails got the idea and took it from her to get a closer look, still pumping his cock with one hand idly. "Wow! Thanks, Amy!" he exclaimed as he peered at his own distorted reflection in the metal surface.

"If you want to thank me, bend over the desk," she told him bluntly yet in a flirtatious tone, gesturing towards the desk with one hand while the other tugged up the front of her skirt, her cock already starting to stiffen up.

Tails was quick and eager to obey, pushing his chair back. "Sure thing, Amy!" he exclaimed as he got up from his seat and immediately bent over, sprawling his upper body across the desk and knocking some of the things on it aside, keeping the plug held in his left hand. He lifted his tails up for her without being prompted, spreading his legs a bit to give her easier access, his hand stroking back and forth over his cock at a steady pace as it pointed down under the desk.

"You've been looking forward to this, huh?" Amy asked rhetorically as she gripped Tails's ass with both hands, grinding her hardening shaft up against it.

"A little, yeah!" Tails admitted, though it was clearly an understatement. "It felt so nice last time, and I've been feeling really good ever since, so... I wanted to see what'll happen if you do it again!"

Amy smirked and reached down, rubbing his balls with one hand for just a few moments. "Don't worry, you'll find out. And remember to keep the plug in for a couple hours afterwards, yeah?"

"I will!" Tails promised, lifting his butt up insistently and grinding it back against Amy's dick. "I waited like you said before, uh, doing some analysis. You won't believe what I found! It turns out-"

Tails didn't get any further as Amy wedged the tip of her cock up against his tailhole, making him moan needily. A far more effeminate cry escaped him once she actually began to penetrate, making her groan out as well, and she slid half of her cock into his ass as she had before, holding onto his hips with both hands. She wiggled her hips just a little to make sure she was good and snug, then gave him a pat on his rumpcheeks. "Hush now, and keep still."

"Okay, but um... could you go a little deeper?" Tails asked, his ears splaying a bit as his voice dipped submissively.

Amy's hold on his ass tightened. "Like thiiiiiis?" she teased, sliding a bit further in so that she had about three-quarters of her cock lodged in his ass.

Tails's moan was even higher pitched this time and his tails both lashed about, his eyes rolling upward for a moment. "Ohhhh...! Y-yeah, just like that," he said before panting softly, stroking his hand faster along his dick.

Amy inhaled deeply and kneaded his butt with both of her hands as she relaxed. "Here you go, then. Remember to keep it all in!" she teased, breathing again until she felt her release starting. "And don't move!"

Tails did his best to obey her command, keeping his hips still – just not the rest of him. His upper body squirmed and he moaned into his arms as he felt Amy's hot piss starting to flow into his ass, spraying his inner walls and filling him with warmth from behind. His tails both swung and lashed about more vigorously over his butt, and Amy leaned back a little so they would stop brushing against her face. His hand jerked his shaft even faster, practically a blur along the length of his dick, his eyes closed as he moaned from feeling Amy's urine pouring into his ass.

This time, Amy wasn't content to merely stand behind him, and bent over as well, pushing into him just a tiny bit more as she slid her hands up his back, his tails sticking out to either side as they waved and wagged. "I've been saving this up for you all morning," she crooned to him as she reached around under his squirming upper body, savoring the feeling of unleashing her bladder into his tight hole.

"A...my..." Tails groaned, panting for breath while jerking his cock furiously. Amy felt over his chest, running her fingers through his white fur; the bumps on his front were small, barely even qualifying as A-cups, but it was a start. Leaning in closer, Amy pressed her breasts up against his back while feeling over the very beginnings of Tails's own.

"Just relax..." she told him, smirking to herself. "Let me finish..." She stayed where she was, the heavy spray of her piss into Tails's ass flooding his guts even more, and he couldn't take it much longer. She heard him moan and whine in pleasure, his tails lashing as his rapid pumps along his cock brought him to climax.

"Amyyyyy!" Tails cried out, his ass clenching around her cock as he came. He writhed under her, and she moved her hands back to his ass to hold him in place as he blew his load all over the floor, arching his back as he moaned loudly. He shuddered under her, his knot inflating and his shaft throbbing and spurting ropes of his seed under the desk, while Amy smugly kept hold of his hips as she finished emptying herself into him. As Tails lay panting and gasping in recovery after coming down from his peak, Amy clenched her muscles to fire a last couple of squirts of pee into him, then slowly pulled out, making Tails gasp.

"Now then," she remarked calmly, reaching over and plucking the butt plug out of Tails's exhausted grasp, "Remember to keep this in for at least a couple of hours. However else you use it is up to you – this toy is a gift!" She then wedged the tip of the plug up against his stretched hole and slipped it in with relative ease. Tails gasped again and moaned, his tails lifting up reflexively, but he remained sprawled on the desk, panting.

"G... got it," he answered after a moment, groaning as Amy pushed on the cap of the plug to make sure it was in snugly, the metallic bulb of the plug resting against his prostate.

She chuckled, then turned around and headed for the door, grabbing her still-hard cock along the way. "I'll be back in a couple of days, Tails! See you soon!" she called out, casually stroking at a quick pace.

"See you, Amy...!" Tails replied with a hopeful tone in his voice. The door closed as Amy left, leaving him panting in recovery, savoring the warmth inside of his ass.

--------------------

Every two days, Amy returned again for another visit. Tails's horniness remained perpetually high, and on the next visit there were cum-soaked tissues and paper towels lying all around his desk. When Amy came in, Tails was already bent over the desk and waiting for her, eager for another 'dose' from Amy.

It was certainly having its intended effect. Every time Amy came by, Tails had changed a little more. The fox's chest grew over time, gaining more volume, the nipples growing larger as well as he grew a proper pair of breasts. It wasn't just his moans that grew more effeminate, but also his voice, to the point where Amy started thinking of tails as a 'she' instead. By the time Tails's breasts had reached C-cups and her hips and ass had rounded out nicely, Amy didn't really NEED to keep peeing inside her any more – she knew that at that point, Tails's body was going to be producing the same hormones as those in her urine by itself. But she kept coming by anyway, partly for fun, and partly because of how much Tails begged for it on each visit. Who was she to deny Tails what she really wanted?

The research Tails was supposed to be doing had been forgotten quickly. The first few visits after the second one had included some half-hearted attempts by Tails to do proper investigation, but eventually Amy came in to find the paper on the clipboard had been covered with doodles of a stick-figure hedgehog much like Amy and a two-tailed stick-figure fox engaged in various carnal acts. The next time Amy stopped by, Tails's whiteboard was covered in more of the same. It didn't take long for Tails to progress from masturbating upon Amy's arrival to being already soaked in cum when the pink hedgehog arrived, making use of the plug Amy had given her for extra fun.

One month after the first visit, Amy came by and received no response when she pressed the doorbell, even when she buzzed it again. A little curious and worried, she tried the door, finding it unlocked. "Tails?" Amy called out as she entered his house. Everything seemed in order, or at least, the robots were still doing their thing, so she headed back to the workshop. Maybe Tails had stepped out and forgotten to lock the door?

The workshop was a mess. Not only was the usual collection of cum-soaked tissues and paper towels strewn around, Tails's desk had been cleared of all its usual contents. It now held a number of sex toys, still slick with lubricant, both dildos and silicone sleeves, with a half-empty box of lubricant bottles beside them. Amy took a step inside, and before she could look around, someone pounced on her from behind, a pair of orange-furred hands reaching around and groping at Amy's breasts while a hard cock rubbed up against her ass through her dress, making her gasp out in shock.

"I've been waiting for you, Amy," Tails moaned out needily. "Sorry for not answering the door, I was busy cleaning up again..."

After getting over her surprise, Amy grinded her ass back against Tails's throbbing canid cock. "If I'd known you were that eager, I would have come by earlier," she teased.

"I want you so badly, Amy," Tails groaned out, grinding against her and pushing Amy towards the nearest wall, the hedgehog obligingly letting Tails guide her. "I've been so incredibly horny. I've already cum four times today and had two showers and I still want more. My hands aren't good enough, my toys aren't good enough. I want you."

Amy grabbed her dress and hiked it up, exposing her ass as well as her half-hard cock. "Go on then, girl," Amy told her, "I'm all yours. Show me what you can do now that you're just like me."

Tails didn't need any further encouragement, and pushed Amy towards the wall. Amy put her hands on it to brace herself as Tails grinded forward once again, hotdogging her pulsing shaft between Amy's cheeks, smooshing her fully-growth breasts up against the hedgehog's back. "Oh, thank you, Amy," Tails groaned out, wiggling her hips as she tried to line herself up with Amy's back door. "I need this so badly..."

Tails pressed the pointed tip of her canid cock up against Amy's ring and began pushing inside, making them both groan out together. She slid in to the hilt, her uninflated knot popping through Amy's sphincter, making the hedgehog moan out and tip her head back, her own dick fully hard now. Tails grinded up against Amy's ass for just a moment before pulling back and then thrusting in once again, taking up a rapid pace immediately.

Amy groaned out as Tails pounded her ass, every heavy thump of Tails's hips against her butt making her cock and balls both bounce. Amy spread her legs out for better stability and felt Tails's own balls slapping against the back of her sack from the power of the fox's thrusts, and Tails was already leaking copious amounts of precum into her from how intensely horny she was.

Tails was moaning too as she thrusted over and over into Amy's tight rear, giving in to the seemingly all-consuming lust she had been enduring that morning. Like Amy had done to her many times, Tails leaned in and wrapped her arms around Amy, groping the hedgehog's tits through the upper part of her dress while railing her from behind.

"Nnh... Amy...!" Tails moaned out as she kept thrusting, the workshop filled with the sounds of Tails's hips meeting Amy's ass. Every moan and gasp the fox gave was completely feminine in tone, and Amy was overwhelmed by how hard Tails was plowing her. Tails's hips were a blur as she humped away at Amy's ass, soon gritting her teeth and crying out, "Yes, yes...! Here it comes...!"

Despite the warning, Tails kept jackhammering at Amy's ass for a few seconds more, her moans getting higher and higher pitched as Amy went crosseyed from Tails's pounding, barely able to hold herself up, her own cock spewing her own cum as she was driven to an orgasm from Tails's intense fucking. Her bouncing dick flung the sticky white fluid all over the wall in front of her and onto the floor as well, as Amy gave a wordless moan in pleasure, her ass tightening up around Tails's throbbing shaft. Amy could barely register Tails's inflated knot hammering against her ass until the fox pushed it in fully, making Amy give a loud cry as Tails moaned out in bliss.

Tails's orgasm was even more intense than Amy's. Tails grinded hard against Amy's ass as she came and held onto Amy tightly, her white-furred balls pulling up in their sack as her cock throbbed within the clenching confines of Amy's hole. Blasting ropes of thick, hot fox cum into the other dickgirl, Tails trembled in orgasmic delight, both of her tails spinning around from the all-consuming pleasure flooding her senses, emptying her balls into Amy from behind. The two of them rode out their climaxes, joined firmly by Tails's knot, until they gradually came down from their peaks and were left both panting deeply for breath.

For a good minute or so they remained there, both panting heavily, catching their breath. "Thanks, Amy," Tails eventually gasped out, "I really... needed that."

"Don't mention it," Amy replied, still a little dazed by her own climax.

"Why didn't... you fuck me before?" Tails asked, pausing and taking a bigger breath. "You always left with a hard-on of your own."

Amy gave a little chuckle. "I wanted this to happen," Amy teased, wiggling her rump against Tails's hips. "And also, I didn't want to interfere with your changes. But now that you're just like me, we can do this more."

Tails nodded and rubbed her hands over Amy's ass. "Jerking off just isn't cutting it for me any more," Tails explained, repeating herself from earlier. "Can you keep coming by?"

"I'll come by every day," Amy promised. "And I'll take a turn pounding you, too. I'll show you just what I'm capable of on top."

Tails gave a little shiver and let out a pleased hum. "I'm looking forward to it," she murmured eagerly, her mind already filling with ideas on what they could do together.