Sheldon looked up as the door to his apartment opened. His blonde neighbor entered, dressed in impractical heels and a brightly-colored dress that revealed indecent amounts of skin.

"Hello, Penny," he said, suppressing a brief flash of irritation because she never knocked, just barged right in. He turned back to his whiteboard, frowning in concentration at his equation. There was something not quite right about it, but he hadn't been able to put his finger on the problem yet.

"Whatcha doin', Moonpie?" Penny asked.

Sheldon jumped as he realized that she was standing almost directly behind him. Normally, his exceptionally sharp hearing made it impossible for anyone to startle him, but he had been so focused on his calculations that he had ignored his surroundings. He turned back to his neighbor with a sigh.

"What are you doing here, Penny? Judging from your choice of attire, I would have supposed that you would be out somewhere indulging your penchant for loud music, questionable males and alcoholic beverages." There, he thought, maybe that would make her go away and leave him to do his work in peace.

She flopped down onto the sofa and sighed loudly. He immediately understood that she would not be leaving anytime soon. Reluctantly, he capped his dry-erase marker, set it aside and gave her his undivided attention.

"The music was fine, the alcohol was great, but the guys were crap. I mean, I still get hit on, of course, but now instead of feeling excited that a cute guy likes me, I find myself wondering what he's really like. You know, if he's a nice guy or not. It's all Leonard's fault," she grumbled.

"I believe Leonard is currently unavailable for coitus," Sheldon offered, not sure what he was supposed to say in this situation. Small talk always baffled him.

She snorted. "Actually, I believe he's coitus-ing Priya's brains out right now. And I don't really want Leonard back. I knew our relationship was a mistake almost from day one. I just wish I could meet someone I could trust, you know?"

"I am very trustworthy," Sheldon asserted, slightly affronted.

"And so not date-able," Penny quipped wryly. "I mean, you've been hanging out with this Amy chick for a while now, and not the slightest flicker of interest. Aren't you ever curious? You never wondered what it would be like to kiss her?"

Sheldon's eyes flitted to her red lips for a fraction of a second. "No," he said and turned back to his whiteboard. He hated it when conversations got personal, and he had no intention of discussing Amy with Penny. The fact that Amy had recently been hinting that she wanted to alter their relationship paradigm was frustrating enough. He enjoyed spending time with Amy, as she was a like-minded individual, but why was she so intent on making their friendship into something else?

Penny watched him erase part of his equation and write down something that didn't look like either a letter or a number.

"So I know better than to ask what you're doing home on a Saturday night, but what are you working on?" she interrupted his train of thought again.

Sheldon sighed as he turned back to Penny. Apparently, she was in an odd mood if she preferred his company to her usual dubious exploits. He began to explain his latest research in detail, hoping she would decide to go watch her trashy reality tv shows or maybe just fall asleep from boredom. She did listen for a few sentences, then held up a hand.

"Wait, I just need to go get one thing," she said. She hopped up off the leather sofa and whisked across the hallway. She opened her apartment door and ducked inside, only to re-emerge a few minutes later with a bottle of wine under one arm and a wineglass in her hand. She sank down on the sofa once again, poured herself a glass and took a large swallow.

"Better," she said happily. "Carry on." She waved a hand at him as if to grant him permission.

Sheldon huffed and put his marker down once more. He walked over to the couch and sat in his 0,0,0,0 spot, next to Penny. If blunt rudeness combined with an in-depth discussion of theoretical physics couldn't get rid of his attractive neighbor, then he could at least make himself comfortable. Now that he thought about it, sitting in the spot that was the center of his universe, discussing physics with an aesthetically-appealing and attentive listener... under other circumstances, he might consider it an ideal evening.

Ten minutes later, Penny was still staring raptly at him as he concluded his explanation of super-string solids. He was about to talk about the next part of his research when she interrupted him.

"You really have great bones, you know that?" she said, running a finger along his chin. "I'm a sucker for those high cheekbones and a strong jawline. It's just so masculine and sexy."

He felt a quiver run through him at her touch and quickly brushed her hand away. "Penny, have you been listening to a word I've said?" he demanded.

"Sure have, sweetie... no idea what it means, of course. You have such gorgeous blue eyes."

Sheldon eyed the wine bottle suspiciously, noticing that it was half-empty. "Penny, you've had too much to drink. I think it's time for you to go home."

"Such a waste," she continued, as if he hadn't spoken. "Amy is just the latest-and most persistent-of a long line of women who throw themselves at you, and you never even seem to notice. Don't you like girls, Sheldon?" she asked, leaning closer to him.

"I don't dislike them," he answered, drawing back. At some point in the conversation, Penny had somehow moved closer to him, and now she was definitely intruding into his personal space.

"I mean, what do you think about when you're alone, when you touch yourself?" Her gaze flickered down to his lap. He opened his mouth to deny it, but it was as if she could read his mind. "I know you do. Remember what you told me the first day we met? That kind of thing definitely leaves an impression. Haven't you ever wanted to know what all the fuss is about?"

"You know I don't like being touched," he said severely. He wanted to leap up off the couch and retreat to his room, but she was leaning so close that he felt trapped.

She shook her finger at him mockingly. "Now Sheldon, what kind of scientist makes up his mind about something with testing it with an experiment?"

That answer was easy. "The theoretical kind," he shot back.

She seemed nonplussed for an instant, then said, "Fine. No experiments, then. Let's call it a bet. One kiss, Sheldon. If you can kiss me and say you feel absolutely nothing, I'll... hmm... I'll stop recording reality tv shows on your DVR."

"And you'll stop pumicing your feet in my apartment," Sheldon added.

She shrugged. "Whatever floats your boat, sweetie. For that, you'd better make it a real kiss, at least five Mississippis. Now if I win, what do I get?" She frowned at him, the tip of her tongue protruding from her mouth as she concentrated. "I know! You have to come dancing with me," she said, her eyes sparkling wickedly.

"Dancing?" Sheldon cried in dismay.

"Yup. At a club of my choosing, for the entire night."

Sheldon groaned. He debated with himself about just trying to shove Penny off the couch and out of his apartment, but he knew she was stronger than he was. In the end, it seemed that the potential benefits of her suggestion far outweighed the risks. In other words, the quickest way to get back to his solitary contemplation of the mysteries of the universe was to agree to her terms, which he did, albeit grudgingly.

"Here," Penny said, holding out her glass to him. There was about two inches of wine in it. He eyed it dubiously.

"What am I supposed to do with that?" he asked.

She stuck her tongue out at him. "You drink it, genius. I thought you might want some... you know, to help loosen you up. Remember, our bet says that you have to kiss me. Besides, alcohol kills germs, right?"

Sheldon took the glass gingerly. "Dear Lord, what fresh hell have I gotten myself into?" he muttered before downing the contents. All he wanted was some piece and quiet so he could work on his calculations. Well, fine, he thought. I'll get this ridiculous bet over with, and then Penny will vacate the apartment. He ignored the little voice that whispered in his subconscious that he had actually been enjoying her company before she came up with this absurd wager idea.

He looked Penny over carefully. From what he understood about kissing, which was mostly gleaned from movies and television, he needed to hold on to her somewhere. Perhaps he could place a hand on her waist? He looked at Penny to find her meeting his gaze with a challenging smirk.

"See anything you like?" she quipped.

He growled in frustration, placed his hands firmly on either side of her waist, and drew her closer. She let out a small gasp of surprise, which caused one corner of his mouth to turn up in a superior smile. Their knees were touching now as they leaned toward each other on the sofa. Gathering up his courage, Sheldon leaned forward, pressed his mouth on hers and with eyes tightly closed, started to count: one Mississippi...

Penny angled her head, and suddenly their lips seemed to fit together in delicious perfection. Two Mississippis...

She parted her mouth slightly, sucking on his bottom lip in a way that made stars dance along the insides of his eyelids. He made a small sound in the back of his throat that was very like a whimper. Three Mississippis...

She was winning the bet, and he couldn't let that happen. He had to take control of the situation. So he changed his grasp, one hand sliding down to span her lower back, pressing her closer, while the other slid up to cradle her neck. Her dress left most of her back exposed, warm skin gliding like silk under his fingertips. At the feel of her skin, his eidetic memory gleefully supplied the image of Penny wearing nothing but her soup tattoo. Penny moaned as he pulled her even closer, and her hands wound around his neck.

Sheldon felt something soft and warm caress his lower lip. It seemed only natural that he open his mouth a little more in response, and Penny's tongue darted into his mouth. The sensation was exquisite, sending waves of heat straight to his groin. He kissed her back, hesitantly joining her in this intriguing new game.

She broke off to trail kisses down his neck, biting gently over the pulse point. This time, he was the one who moaned. She shifted, moving to straddle him as he sat on the sofa, and now he found that his hands seemed to rest naturally along the outsides of her thighs, and he began to work his way upwards until he pulled her hips tight in contact with his groin. She made a little whimpering sound and ground against him as she recaptured his lips with hers. He gasped her name, and his body began to quiver as the tension built within him.

The need to see and touch more of her soft curves filled his thoughts. He pushed her dress's thin straps off her shoulders, and a quick tug exposed her breasts to his hungry gaze. He traced her curves with his hands, and she arched back with an inarticulate cry as his fingers grazed the taut peaks of her nipples. His mouth followed where his hands led. It was strange new territory for him, but her increasingly loud moans urged him on. He teased the rosy buds with his mouth and hands until she was shaking with need, her nails digging hard into his upper back. She rose up, swiftly undoing his fly and belt buckle and pulling his pants and underwear down below his knees.

She whispered his name, and one soul-searching glance into his eyes gave her the answer she was looking for. She stroked his shaft with one hand, kissing him deeply as she lowered herself down onto him one tortuous heated inch at a time. He bucked his hips forward, and she gasped, feeling her insides stretch to accomodate his girth. She clenched her inner muscles around him involuntarily, and he began to murmur her name like a mantra as she rocked up and down. Taking his hand in hers, she guided his fingers to find her pleasure center. Her loud expletives as he touched her sent his own excitement soaring. He was sure he wouldn't have been able to stop what he was doing if the apartment had been burning down around his ears. He could feel the crest of his orgasm building, and he redoubled his efforts. In a few more seconds, she was screaming his name. The feel of her inner walls pulsing around him caused the world to fly apart and fade into blackness.

When he opened his eyes, he was astonished to find that he was still in his apartment, on his couch, in his 0,0,0,0 spot. He felt as if things had changed so drastically that he found it hard to believe he was in the same universe anymore. Penny's head drooped against his shoulder, and she shuddered as the last aftershocks of her climax pulsed through her body. She clung to him, kissing his neck.

"Oh god, that was incredible," she whispered.

"I don't think even winning the Nobel Prize could compete with that in terms of sheer ecstasy," he murmured, feeling dazed and satiated in a way he had never experienced before.

Suddenly, she began to laugh, her body vibrating against his chest.

"What do you find so amusing?" he asked, although he was feeling so relaxed, he could barely muster the energy to care.

"That was only three Mississippis, babe," she said lazily. "Imagine what five is like."

Sheldon's eyes sparked with interest, and he captured her lips with his once more. "I think further experimentation is definitely in order."