Sheldon came knocking on her door the next evening. As she answered with a cheery smile and "What's up, buttercup?", she wondered how she could be so happy to see him when she felt that stab of remorse at the same time. As time passed, she could almost forget that shameful moment when she woke up next to him. Thankfully, Sheldon showed no signs of remembering anything about that night. But her guilt didn't stop her from going over to his place the next time he came knocking on her door. These days, they spent almost all of her nights off together, and it was often just the two of them. Leonard was almost never home. Penny wondered when Sheldon was going to realize that his roommate had one foot out the door.

"Come on in," she invited. She hadn't yet been able to convince him that he didn't need to ask permission to enter. Then again, she was pretty sure he just liked knocking three times while saying her name over and over. He always had this adorable, self-satisfied smirk on his face when he finished. She almost groaned aloud as her mind instantly combined the idea of repetition, him calling out her name, and a satisfied Sheldon into something far dirtier.

Get your mind out of the gutter, Penny mentally scolded herself. Although she couldn't remember a thing about the night they spent together in Vegas, she had become consumed with curiosity to know what kind of lover Sheldon might be. The only thing she knew for sure was that they had actually done the deed, since the next day, she had a slight ache in her lower back. That meant one of two things: either Sheldon had been very enthusiastic, or in a locker room situation, he would put most other men to shame. Thinking about either of those two possibilities made it hard for her to look him in the eye.

"I have season two of Battlestar Galactica," he said, holding up the boxed set. His eyes sparkled at her, and she knew he was eagerly looking forward to watching it with her. And she found she was willing to do all sorts of things she wouldn't normally do to see that happy light in his eyes.

"The pizza should be here any minute… sausage, onions, and light olives," she said before he could ask if she had gotten the order right. Of course, she'd gotten it right; she was a waitress. The only time she messed up people's orders was when it was deliberate: when they were driving her crazy, and she knew she wasn't going to get a good tip anyway.

"I suppose Amy has already informed you that she's going on a date with Stuart," he said as he sat down on the left side of the love seat. He had finally decided that was his spot in her apartment, although it was understood that it could never compare to his "center of the universe" on the brown leather couch in his living room.

She sat down next to him, her brow creased with concern. "No. I mean, we talked about it a little, but she didn't say she was actually going to do it. Are you okay, sweetie?"

He sighed. "I don't like the idea. If she starts dating Stuart, then she will no longer be available for our regularly scheduled meetings. I'm well aware of my friends' tendency to prioritize coitus over spending time with me," he added glumly.

Penny wanted to laugh at that statement. On the surface, it was ridiculous. Of course, any normal person would choose getting laid over hanging out with him. On the other hand, his friends shouldn't totally ditch him the way they did either. Leonard now spent the night at Priya's so often that he had somehow bribed or coerced Raj into giving Sheldon a ride most mornings. He barely even saw Sheldon anymore outside of work.

"You are a curious exception," he said, and her attention jerked back to the present.

"Huh?"

"Of all my friends, you are the most successful at pursuing coitus with various partners, yet you spend most of your evenings off with me. I do appreciate the effort, although I must confess, your motives make little sense to me."

"Honey, it's not an effort. I like spending time with you. But this thing with Amy... she just wants to be your girlfriend, you know."

"I have been acutely aware of what Amy wants for months," he grumbled. "However, it's not possible."

"It's not?" Penny asked. How could such a simple statement fill her with so much hope and dread at the same time?

Just as he opened his mouth to respond, there was a knock at the door.

"Pizza delivery!" yelled a voice from the hallway.

She glowered at the door as Sheldon got up to pay the woman who had brought their food. She desperately wanted to know what he had been about to say, but he immediately cued up his sci-fi series. She didn't have to know much about Sheldon to know he wouldn't appreciate her interrupting his show.

She kept glancing over at him throughout the episode. Why did he say he couldn't ask Amy to be his girlfriend? Did he have feelings for someone else? And wasn't she the only other woman in his life? Maybe he'd just decided that he was never going to be involved with anyone, ever. She needed to find out, and the now-familiar stab of guilt that she felt every time she saw him wasn't going to stop her.

Sheldon had been oblivious - perhaps deliberately so - about Leonard's absences from the apartment. When a roommate spent most nights of the week at his girlfriend's place, most people would expect that he would soon move in with her. Sheldon was not most people. He didn't come to that conclusion on his own, but had the knowledge thrust upon him suddenly and unpleasantly. Penny got a text from him late one night at the end of her shift. All it said was, "Come over whenever you get home." She raised an eyebrow at that and ducked into the ladies' room to give herself time to think. Sheldon had a strict bedtime, and it was already past that hour. If he was breaking his habit, something must be up.

Her phone rang while it was still in her hand. She startled and almost dropped it. When she answered, the caller ID showed it was Leonard calling. Immediately suspicious, as he had barely spoken to her in weeks, she answered and soon found out why Sheldon had sent her the cryptic text. Leonard was moving in with Priya, and he had just broken the news to his roommate - and supposed best friend - earlier that evening. Priya and Leonard had gotten much closer in the past few months, so when she was offered a promotion that required her to move back to India, she had turned it down in order to remain in the States with her boyfriend. Leonard hinted that they might even get engaged soon. Penny wasn't clear on the details, but it didn't matter. She knew Sheldon would be devastated, and that was all she cared about.

She left work as soon as she could and sped home. She found Sheldon curled up in a ball in the center of his bed, much like he'd done when he found out Leonard had sabotaged his Arctic experiment. Guilt stabbed at her again. Both she and Leonard were his closest friends, but the way they treated him, she wasn't sure either of them deserved that title.

She sat down on the edge of the bed and touched his shoulder tentatively. "Leonard told me he's moving out. How are you holding up?"

"My entire life is descending into chaos," he muttered, turning his head to glare balefully at her.

"Oh, come on, sweetie, it's not that bad. I'll still be right across the hall. We'll still have Halo nights and stuff, and I'll come have dinner with you when I'm not working."

"That's hardly the same as having a roommate who's been your friend for almost a decade. Besides, I have other… issues that you don't know about."

She rolled her eyes, even though he couldn't see the gesture. "Sweetie, everyone knows you're a little different. I'm still your friend, and so is Leonard."

He sat up in bed and turned to face her. "No, I made a mistake," he said haltingly. "I have been… obsessed with solving the mystery of how I could possibly have done… what I did."

"What do you mean, sweetie?" Penny asked, although she could feel her heart start to pound in her chest.

He looked around furtively, although there was clearly no one else in the apartment. "Do you remember the weekend we all spent in Las Vegas?"

Penny began to feel faint and dizzy. "Uh... yeah," she managed to choke out.

"Do you recall seeing me in the company of any female other than yourself?"

"No, I don't think so," she answered, as a blush stole up her throat and heated her cheeks. "Why?"

"Wait here," he ordered. He strode to the door of his room, poked his head out into the hallway and looked around. Then he closed the door and locked it. He opened the top drawer of his dresser and carefully removed a wooden box from the very back corner. It was apparently a puzzle box, as he spent several seconds pressing, turning and flipping various parts of the box. Penny watched him curiously, wondering what on earth he could be so secretive about. Finally, the box sprang open, and he withdrew something small which he concealed in his hand. He sat down hesitantly next to Penny and then opened his hands to reveal a jewelry box. "Please do not misinterpret this gesture," he said solemnly, opening the box.

She gasped as she saw a sparkling diamond band nestled against the black velvet.