A friend and I were discussing Penny's lack of a last name one night and we stumbled upon this explanation.

:The Secret of Penny:

Despite what his friends, family and co-workers thought, he had plenty of secrets. Small secrets that all stemmed from one big secret. One huge secret that burned in the pit of his stomach every minute of every day. His secret was that he knew her secret.

How she thought to keep it from him, he didn't know. This was something he had dreamed of all his life. Something he thought he was prepared for. Until it happened. Okay, so he hadn't understood it at first. To his chagrin, it would be a full year before he actually figured it out. Once he had, it made everything else make sense.

Suddenly he understood why she worked as a waitress. He understood her fascination with acting. He knew why she found them all so interesting. It had made no sense why a beautiful woman would willingly hang out with socially inept, geeky outcasts like them. Unless you knew her secret.

Of course, knowing her secret raised more questions. Should he tell her he knew? If yes, when? If no, what did he do then? In the end, he chose to not tell her. Instead, he chose to protect her as best he could. Sometimes she got exasperated, even angry at his actions, but he did them for her benefit.

He tried to push Leonard in a direction that would appeal to her. He brought her into his life and routines, so he could be there to help in anyway possible. Helping her with her finances, offering advice, cleaning her apartment on a monthly basis, and generally keeping the others from realizing the truth.

He also kept her from knowing the truth also. He employed the use of snide remarks, veiled insults and disparaging remarks to make sure she never knew the intense scrutiny he kept on her, or the high regard he held her in.

Now he wasn't sure what to do. Leonard had pushed her away by trying to hold her too close. She was no longer a member of his social circle. He had no idea what she was doing now. He had only seen her once, and he had been with Leonard so she didn't stay to converse.

Sheldon paced his room. What if she left? What if her time here ended and he wasn't around to know it? That was unacceptable! This was Wil Wheaton's fault. Why did he have to interfere? Sheldon pounded his fists against his thighs. He had to figure out a way to return everyone to their previous status.

He looked at the clock, seeing it was only just midnight. Maybe she was still awake. He had to talk to her. He was crossing the living room as quietly as possible, so as to not alert Leonard to his actions, when he heard Penny's door open. He paused and heard light footsteps going up the stairs. He waited until the sound faded and then slowly followed.

He peeked around the corner of the roof door and gasped. Penny spun fearfully, but relaxed when she saw him. She smiled and Sheldon stepped out onto the roof. He approached hesitantly and she laughed lightly.

"I knew you at least suspected," she said, answering the question in his mind. Sheldon's eyes widened, the whites in stark contrast to the darkening blue of his iris.

"You can read my mind?" he asked. Penny shook her head and laughed again. Sheldon deliberately tried to send a thought her way, and she just giggled more. He frowned, knowing that his thought should have angered her. Penny really disliked it when anyone commented on her mannish hands.

"I can't read minds, Moon Pie. Just your expression." Penny sat down on an upended crate and watched him.

Sheldon couldn't resist the openness in her expression. He drew closer until he was just a few inches away. Slowly he walked around her, taking it all in. Her eyes were still hazel, only it appeared that there was no pupil or sclera. The entire eye was a crystalline green, with flecks of black in it. Her ears were set higher on her head, and they had a double notched lobe. Penny reached up and pushed her hair back behind her ears and he noticed there were very thin aqua streaks in it now. Her skin was golden and smooth, but odd somehow. It took him several second's to realize why. It wasn't her skin that was golden, it was the veins under the surface.

He leaned in for a closer look and could easily make out the network of veins under the translucent skin. It was as if she had thousands upon thousands of miniscule golden threads embedded in her epidermal layer. He tentatively reached out and traced a clustered at the base of her jaw. Sheldon hissed lightly at the iciness of her skin.

Penny giggled. "Sorry, I should have warned you. In my true form, my body temperature is about 8° F."

Sheldon gingerly sat down on another crate facing her. "Where are you from?" he asked respectfully.

Penny shook her head and laughed. "Keep that up and I'm not telling you anything. I'm still your friend, still your neighbor. Treat me like it, Moon Pie"

Sheldon gave her a faux glare. "Only Meemaw calls me that."

"Much better," she laughed. "My species is from a planet in a solar system your world hasn't discovered yet. It's about 230,000,000,000,000 of your light years away. My species calls it Ymarb Lanae. It circles a blue sun, fifth in orbit of 11 planets. It's mostly water like Earth, but it's a lot cooler there than it is here. Our temperatures range between 35° and 40° on average for summer."

"That is why you always dress in so little," Sheldon said with realization. "You are always too warm here."

Penny nodded. "It's always so sweltering here. Almost makes me wish I could take on a real human form all the time, but it's exhausting. I have to revert to my real form every 48 hours or so in order to recharge. Of course then I feel the heat even more and it's stifling."

Sheldon cleared his throat. "Penny...," he hesitated and looked down at his hands.

"Soon," she said softly. "I still need to make the final arrangements."

Sheldon twitched and wrung his hands together. "I have dreamed of meeting an alien ever since I was 4 yrs old. I always imagined it as a meeting of the minds. Someone I could discuss my theories with, who I could talk about the mysteries of the universe with."

Penny's eyes turned sad. "I am sorry you are disappointed." she said.

Sheldon looked up in surprise. "But I am not," he declared. "As interesting as that would have been, meeting you has not been a disappointment. You are my friend, Penny. I …" Sheldon blushed slightly. "I like you. I will miss you." He swallowed the lump in his throat. "I don't want you to go," he said murmured.

Penny stood and he watched in awe as she shifted back into her human persona. He was actually disappointed, thinking her real form was more beautiful. Penny walked over and held out her hands. Sheldon placed his in them and stood.

He watched her search his face, for what he had no idea. Slowly she leaned up and pressed her lips to his. Sheldon gaped as pleasure rushed through him. His eyes drifted closed as the taste of her cherry lip gloss exploded in his mouth. He had suffered through kisses before. Many of them. But none had ever been like this. His arms moved of their own volition, wrapping around her waist. Hers slid around his neck. Warmth spread through them both as the stars twinkled overhead.

Sheldon looked at himself in the mirror and took a few deep breaths. Was he really going to do this? He opened the bathroom door and walked across the living room. He paused for only a second to see Priya and Leonard by the door. Why was Priya here? And why did it appear as if she was leaving? Sheldon shook his head. He had other things to think about.

Um, it's not what it looks like," Leonard said quickly.

"Okay," Sheldon said as he moved past them and out into the hall. He crossed the hall and knocked on Penny's door.

He was beginning to get nervous by the time it opened. He could feel Priya and Leonard's eyes on him. Penny was yawning as she opened the door. She looked up at him with bleary eyes.

"What's going on?" she asked grumpily.

Sheldon licked his lips and almost sprinted back into 4A. Instead he took a deep breath and leaned down. He kissed Penny softly and immediately felt her relax. He smiled against her lips and felt her do the same.

He pulled back and blushed. "Good morning, Penny."

Penny smiled softly and pecked Sheldon's lips. "Good morning to you, too."

"May I come in?" Sheldon asked. Penny grabbed his belt and pulled him into the apartment. She shut the door on the astonished faces across the hall and turned to Sheldon.

"What's up, sweetie?" she asked, heading into the kitchen to put on a pot of coffee.

Sheldon gathered his scant amount of courage. "I want to go with you." He watched Penny freeze and his courage faltered a bit. "When you leave, I want to go with you."

Slowly Penny turned to face him. "So you can learn all about the universes?"

Sheldon took a deep breath. "No. Because the thought that I may never see you again is painful to me," he admitted. "Because I want to be where you are. If," he paused and moved closer, his eyes begging her in a way his words couldn't, "if you can't take me with you, will you stay here?"

"You want me to stay?" she asked carefully, seeking clarification. "To be your friend?"

Sheldon shook his head. He reached out and caressed her cheek gently. "To be with me. My friend, my" he blushed deeply, "everything."

Penny grinned suddenly and leaned up to kiss him. Sheldon wrapped his long arms around her quickly and held her tight. "Please," he whispered against her lips. "I don't want to lose you."

Penny grinned. "I don't want to lose you, either."

Three weeks later, Penny and Sheldon went for a drive. She parked her car on a small dirt road outside of town. She had told him he could only bring what he could fit in his messenger bag. He had his Spock napkin, his letters from Meemaw, photos of his family, and his Kindle loaded with his favorite comics. His only regret was that his family and friends would never know what had happened to him and Penny.

Yes, he was going to travel among the stars, see new worlds, meet new species. More importantly, though, he was going to be with Penny. Strange how the only woman in the world that he had ever loved wasn't even from this world. They waited in a small clearing for their transport, hand in hand.

Penny looked up at him, her hazel eyes shining brightly. "Last chance," she said softly. "You can still change your mind."

Sheldon shook his head. "My choice was made the day we met, Penny. I choose you."

A small sleek craft settled noiselessly in the clearing. A door slid open and two males walked over. One was a male humanoid, with golden skin like Penny's. The other was pasty white. His hands had eight fingers each and his skin was covered in short needle-like spines.

Penny smiled at Sheldon and pulled him toward the craft. "Let's go, Moon Pie. I'm going to show you my world, now."

Sheldon smiled and followed her aboard. "As long as I see it with you."