Authors note: I wrote about what I would have like to have happened during the Season 3 episode The Gothowitz Deviation. Only in my version Leonard and Penny aren't dating and Penny is staying there because in her usual,Penny like way, she jammed her key in her lock (or she forgot it. Minor tangents in the story). So, I got to thinking, "What if Sheldon reacted in a different way when he saw Penny in his shirt (I'm going to assume it is his because I don't see how it could be Leonards. It wouldn't fit her right if it was Leonards IMO). And, viola here is a ficlet.
One more thing. What does one use as the medical name for your jawbone? Is it corpus mandibulae of just mandible? I wrote mandible but if it's wrong feel free to correct me.

He shuffles sleepily down the hall, rubbing his eyes furiously. He hates this interruption to his REM sleep cycle.

As he walks into the kitchen, he is taken back by the sight of a perky Penny clad only in a long sleeved shirt and red panties. His shirt to be precise. The arms are rolled up and the hem barely covers her…butt.

She is doing some ridiculous things with her arms and legs and belting at the top of her lungs to Shania Twain.

The sight of her in his shirt causes a weird, tightening sensation on his chest. Not in any of his infinite number of universes can Sheldon imagine seeing Penny like this. It unnerves him, leaves him feeling embarrassed and something else he can't identify.

This unknown feeling, unknown variable irritates him. He doesn't understand and this rattles him.

He watches her move around the kitchen, watches the shirt expose then cover her red clad butt. He notices the long, tan legs and the messy disarray of blonde hair curling around her shoulders.

She turns, gives him a big grin and says, "Come dance with me."

He's managed to regain control of these unidentified emotions and states, briskly, "No."

She gives him a mock pout and his dark gaze is immediately entranced by her full lips. So pink. So full.

He takes a small step back and says, "Is it really necessary for the music to be at such a high volume?"

She leans over, pressing the pause button. As she does, he is once again treated to a view of Penny's rather robust butt.

"Come on Sheldon. Have fun. Let loose a little."

She gives him another smile as her heart begins to pick up speed. She's noticed his appraisal of her. She's felt his dark gaze on her back, on her ass, on her legs. Hell, she felt those blue eyes on her lips too.

He may not recognize arousal but she does. Oh, yes she does. She looks into his darkening blue eyes and notices the way he nervously wets his lips and watches the way he fingers the material of his robe.

She's surprised by her response to him. She's soaking up his reaction to her. In fact, she's suddenly developed this urge to kiss Sheldon Cooper. She wants to lick at his bottom lip, pull on the soft skin.

Wait just a damn minute. She wants to kiss Sheldon?

Not Sheldon, whose arrogance and condescending nature irritates her day in and day out. But, all the signs are there and Penny wonders what would happen if she acted upon these startling emotions.

She looks at the nervous man before her and knows exactly what would happen.

But Penny's curious now. How many times has Sheldon Lee Cooper been thoroughly kissed?

"Sheldon, have you ever been kissed before?"

His eyes widened as a light pink tint forms along his cheeks.

"Penny, I'm certain this not an appropriate topic of conversation we should be engaging in."

She looks at him steadily, letting him know she's not going to let this rest.

He grimaces slightly and says, "You remember Leonard's mother."

"Ah, yes. Did you enjoy it?"

"No. She tasted like alcohol and she pressed too hard against my lips."

"How about before her?"

"When I was younger, although that experience wasn't any better than the one after it."

So, he's never had a proper first kiss. Well, she intends to give him one.

She may not possess two PhD's in science but what she does have is PhD on the subject matter of kissing.

He's standing in front of the refrigerator, looking fidgety and quite adorable.

She eyes him thoughtfully. How to proceed from here?

"Sheldon, sweetie, what would you say to a little experiment?"

He regards her warily. Judging by the way she is looking at him, he's starting to realize that this experiment she has in mind isn't something he'd willingly do.

"What type of experiment? What are the variables? What equations do we need-?"

She's eyeing him like a cat would a mouse. A light sheen of perspiration coats his forehead as she starts advancing upon him. Slowly and dangerously.

"P-penny?"

"Just relax sweetie."

Relax? How can he relax when she keeps invading his space?

She's got him cornered now. He focuses on a spot on the wall above her head, trying to control his body awareness of her.

She tilts her head back, her eyes settling on his lips. Tentatively, she places her hands on his chest. She's surprised at the hardness she feels beneath her hands. Sheldon may not have big, bulging muscles but he's definitely not soft.

"Sheldon, look at me."

He glances down at her and nervously licks his lips.

"To close."

"It's an experiment."

"I don't like this experiment."

"We're friends right?"

"Yes, but-."

"You trust me right?"

He does trust her. He trusts her more than Leonard, his own roommate. She's become an integral part of his life and there's this sense of kinship between the two of them.

"I do."

"Good."

She stands on her toes and places her lips softly against his.

He stiffens at the contact, hands frozen at his sides, eyes wide open. He feels her lips moving against his own and he feels that he should respond but is uncertain about how he should.

She moves her lips to the side of his neck and whispers, "Trust me."

He looks at her as she move her lips along the edge of his mandible and lowers his head to accommodate her.

"Put your hands on my hips."

He hesitantly places his longs fingers on the flesh of her hips and instinctively squeezes. She moans and molds her body to his.

He closes his eyes, finally accustomed to her intrusion upon his person.

Her tongue runs along his teeth, probing, demanding. He opens his mouth, allows her entrance. The minute their tongues touch something comes to life in him.

He grips her hips roughly and loves the way her hands have found purchase in his hair.
She can't believe it. By the way he's gripping her body, you'd think he had done this all his life.

She can feel every inch of him, every damn inch, from his hard chest right down the hardness pressing into her stomach.

Something biologically primitive is igniting the hidden recesses of his being. His hands wander down her sides to cup her ass.

He easily lifts her up, carries her to the counter.

He brings up a hand to tangle in her blonde locks.

She comes up for air and breathes heavily into his neck. Holy crap on a cracker.

"Sheldon," she whispers between gulps of air.

How had this little experiment turned into Penny making out with her annoying, whack-a-doodle neighbor?

She inhales soap and cotton as her fingers bunch into his robe.

This is not the type of experiment he expected so early in the morning.

He eyes the fingers still wrapped in her hair and the hand still cupping her side. The endorphin rush he's feeling is amazing in its intensity.

He pushes away from her slightly and regards her upturned face. Her lips are pink, slightly swollen. Her cheeks are flushed. Her pupils are dilated.

Penny is aroused. By their kissing. By him.

Who would have thought that Sheldon Cooper could arouse her from one simple kiss. It's unnerving to think about yet Penny can't help but think Kiss me again.

"Penny, I-."

Ask for another kiss. Kiss me again.

"I just- I need to go take care of something. I'll be right back."

He hurries down the hall and bumps into a tired looking Leonard.

"Hey buddy. Is Penny awake?"

In more ways than one.

"I-yes, she's in the kitchen."

With a stiff nod he continues to the bathroom, firmly shutting the door behind him.

Leonard isn't fazed by the interaction. When dealing with Sheldon, he just goes with it.
He ambles into the kitchen to find a very disheveled and confused looking Penny.

"Hey Penny what's wrong?"

She's eyeing the counter in somewhat of a dazed way and says, "Just trying to remember what time the locksmith said he'd get here."

"Oh. Feel free t stay as long as you need to."

She thanks him and mumbles something about needing to try another experiment before heading to the living room.

Leonard's confused about her statement of needing to try another experiment. After all, what could Penny possibly need to experiment on?