Sheldon growled as he sat up in bed for the fourth time that night and blearily tried to calculate the equation for the amount of sleep he had lost over the past few nights. He put a hand up to his temple and massaged it, closing his eyes. The variable "x." The one multiplied by its importance and added to the number of nights it had been since he had had a sound night's rest. He bit his lip. Where was that markerboard?

The apartment was quiet, and Sheldon realized it was close upon three in the morning as he snatched up a small whiteboard from its place beside the bookshelf and a marker from the bedside table. It would be probable that solving the equation for sleep will induce it he thought,and he settled to scribbling out the formula from where he sat cross-legged on the floor. It was a pleasing concept and he continued to write furiously, progressively narrowing down the equation to…the variable "x." Damn it all, what was it?

A soft bumping sound shattered his concentration, and carefully placing his work on the bed, he crept out of his room to find Leonard thudding his head against his bedroom door.

"Leonard, might I-"

"No, Sheldon, don't ask," came his friend's reply as he shut the door clumsily behind him.

Sheldon blinked as he realized the lights in the living room were on. He shuffled warily down the hall and into the main room, surprised a little to find Penny and Beverly conversing loudly and… were they intoxicated? The concept of Penny consuming large amounts of alcohol was not an occurrence that surprised him, but Leonard's mother doing so was an unforeseen development.

"Why is Leonard softly banging his head against his bedroom door?" he asked, planting himself before the two women in expectation to the answer to his inquiry.

It happened too fast for Sheldon to give it proper analytical thought. Beverly was saying something about "warm feelings" and taking unsteady strides toward him. Suddenly a body slammed into him and his lips grazed blonde hair. Penny?

Her back was pressed against him, her left hand reaching back to clutch the hem of his pajama shirt while her right hand forced Beverly back from them.

"T-that's enough, Beverly," she commanded, her voice becoming increasingly steadier. "Here's my key. It's past everyone's bedtimes, don't you think?"

"Yes, I suppose so." Leonard's mother turned away and tottered across the hall toward Penny's apartment.

Sheldon had not shrunk back from Penny's pressure against him, but curiously allowed the close contact to endure. His breathing had become shorter, slightly ruffling her blonde hair as it continued to brush against his face. He involuntarily clutched her arm to steady her balance against him, too dazed by her close proximity to think better of it.

Beverly had left and suddenly Penny whirled to face him, and the bond was broken. "I'm so sorry, Sheldon. I had not idea she would try something like that."

Sheldon blinked. "Try what?" It had occurred to him that Leonard's mother had the intention of stepping close to him, but to what purpose?

"She was going to kiss you."

Sheldon cocked an eyebrow and looked down at her. "Penny, there is no possible way you could know that. Did she tell you? What facts do you-"

"It was an inference, Sheldon," Penny replied almost irritably in her drunken state. "I saw it coming, but I just…I just couldn't let it happen."

"Why not?"

Penny stared at him. "Because you couldn't, you brilliant scientist you, calculate the result of that equation. You wouldn't have liked the solution."

Sheldon couldn't help but smile a little. "An accurate conclusion, Penny. I am very impressed."

Then she was hugging him, clutching at his clothing and resting her cheek against his chest. Sheldon's breath hitched as she once again pressed herself against him, but this time he tentatively wrapped his arms around her shoulders in return.

"It would have…damaged you," Penny whispered haltingly. "It would have…she…she wasn't the right person."

Sheldon's grip around her tightened. She had saved him, certainly. She had cared enough to notice the problem and was concerned about how it would have affected him. She knew him. Something swelled inside him. Pride? Affection? It was only when he felt his lips brush her neck that he snapped back to reality and slowly released her.

"Thank you, Penny," he managed in a faltering voice. He winced. What is happening to me?

Penny nodded and patted his cheek with a smile. "Certainly, sweetie." She turned to leave, unsteady still in her inebriated state until Sheldon reached out and grabbed her arm.

"Penny, who is the right person in…in this equation?" he asked quietly. He was never one to ask anyone how to solve anything.

Penny grinned as she gently pried his fingers from her arm. "I don't know, Sheldon. You'll have to figure that variable out for yourself." And with that she was gone, closing the door firmly behind her.

Sheldon stared at the door, frozen. The variable. She's my missing variable.