She can't help herself

Serena whimpered, looking up at you, but then dove back down onto your cock, and you knew she'd made her choice. Immediately, the extremely pleasant sensations she was giving you started to change in subtle ways. You almost stopped her for a moment to watch what was happening, but she gave a particularly enthusiastic twist of her tongue and you decided that could wait.

From your slightly propped-up position, you watched her blonde head bobbing up and down, and you both saw and felt it as her lips plumped up. You both groaned at the feeling, and she started making little noises that sounded pleasurable - if you had to guess, you'd say her mouth was starting to feel good to her too. Her skilled tongue, already engaged in flicking around your shaft while she kept sucking, suddenly felt like it was somehow giving a more intense sensation.

You weren't sure exactly what other changes were happening, and nothing especially new happened for a moment, so you just sat back and enjoyed her ministrations. Then, she paused for just a moment. She was still looking up at you, her mouth wrapped around your cock, of course, but there was a slightly confused expression in her eyes. You were just about to stop her and find out what was wrong, but then she lowered her mouth further down around you again, all the way down to the base. Her eyes widened, clearly surprised, but you weren't sure why - until she stayed there, not gagging or coming back up, for a long minute. She gave you a pleased little shiver, and her tongue slipped out beneath your cock to lap at your balls.

You grunted, surprised but pleased at this development. "No gag reflex left, huh? Can you breathe down there?"

Serena hesitated, then nodded the very small amount she could with your cock buried in her throat. She slipped her mouth slowly back up your shaft to tease at the tip for a moment, and you groaned again. All of this was officially starting to get be too much, and you were right on the brink. You felt like you should add one more wrinkle before letting her finish you off, though. "Would you rather cum hard every time you swallow, or every time you get a facial?"

She moaned around your cock, and sucked even harder. Her tongue gave that weird twist again, and you practically lifted off the bed as you spasmed and unloaded into her mouth. She never stopped working your cock the whole time, swallowing down what felt like an enormous amount of cum even as her body started shaking with her own intense orgasm.

For a few minutes, you both lay there breathing hard, her head resting on your thigh with your cock still deep in her mouth. You stroked her hair once, and she gave your cock a lick. You twitched, the feeling a bit intense at the moment, and gently pulled her off you. She whined a little, but let you maneuver her up to lay next to you, and you got your first look at Serena's 'improved' mouth.

Her hugely enhanced lips were the first thing you noticed, for obvious reasons. Had she been wearing lipstick before? You didn't think so, but even if she had, now they were a deep intense red, the kind of color that could set a man to thinking about what to do with those lips pretty much every time. They were also much larger and thicker now, plush and luscious. Not "surgery gone terribly wrong" big, but they still dominated her face, like she was constantly giving the world's best duck lips.

You reached over and tapped her lips gently with a finger. She gave a little "mm" noise and opened her eyes, looking up at you. "Do these feel good?"

She wriggled her body against yours a little, giving you a wicked smile. Damn. Her smile had been pleasant enough before, but now it was ridiculously hot. Somehow it just instantly told you the things her mouth could be doing, maybe should be doing, and if it was like that for you now right after she'd just sucked you off...whoof. "Care to show off what else is new?"

Serena nodded eagerly and sat up. With a showy flourish of her hands, she slowly extended her tongue, and deliberately curled it up to touch her nose. She didn't just bend it upward or anything, she turned it around in a circle that had to extend a few inches out from her face. After turning her face this way and that for you to see, her tongue rippled all along its length, and she slowly pulled it back in, showing off her dexterity along the whole thing.

You clapped politely, and she grinned back at you silently. She held up a hand, and then very deliberately took a deep breath, then held it. And held it. Aaaaand held it. After a minute or so, you got the idea and got out your phone to set a timer. As it started ticking away the seconds, you smiled at her. "I get the concept; let me know when it gets hard for you, but otherwise you're on the honor system here. Anything else?"

She smiled and nodded. She shuffled a little closer to you and opened her mouth delicately, pointing to her teeth. They were perfectly straight and movie star white, but that hardly seemed impressive, considering the other changes that had happened to her. She tapped one with a fingernail, though, and it didn't click like you expected. She picked up your hand and slowly closed her jaw over your index finger, biting down gently, and her teeth...bent? You blinked, and wiggled your finger. Her teeth were soft, almost pillowy, against your hand. You chuckled and played with them for a minute, and she playfully nipped and licked at your finger as you did so.

"Wow, that's nuts," you said. "Your mouth really is just meant for sucking now, isn't it? Can you even eat normally with those?"

She cocked her head to the side with an obvious questioning air, tapping her chin with a finger as she thought about it. Then she grinned, shook her head, and suggestively patted your crotch. You gave her your own questioning look, and she rubbed her stomach with a 'yum-yum' sort of movement, then rested her hand on your cock again. Your eyebrows raised. "You...think you can suck cock to eat?"

She nodded enthusiastically, and you whistled. "Damn. That one did more of a number on you than I thought."

You let her lay back down on your chest, and you relaxed for a little while until she tapped your chest, then your phone. You looked down at her, and she tapped your phone again, a little urgently, and you realized the timer was still running. "Oh, crap, sorry, go ahead," you said. She let out a breath and started breathing normally again as you said aloud, "Whoah. Ten minutes, and you're barely even winded. Was that your limit?" She shook her head, grinning smugly, then laid back down and snuggled into the crook of your arm.

You laid back too, tucking your other arm under your head and looking up at the ceiling contemplatively as you pondered what you were going to get up to next.