A/N - So okay I just came up with this yesterday, I had a really hardcore Saturday of staying in writing and watching Friends & TBBT. Anyway I thought I'd share it, I came inside for a bit to break up time spent in the super-rare sunshine in case I burn! 79 degrees in London is a rare occasion! Let me know what you think, and fellow Brits don't forget your Factor 50 ha ha!

Stepping out of the shower and wrapping herself in Leonard's towel, Penny smiled to herself as she drew shut the shower curtain.

For a moment, she observed it. Almost seven years she'd known them, and she'd never once really looked at it properly. She hadn't had that many showers here, and a fair few of them had been with Leonard, so those times didn't really count. But she'd come in to use the loo a few times, or wash her hands, or brush her teeth and wash her face in the morning if she'd stayed the night. Or taken a bath. Either way, she'd never really given it a proper look. It just hadn't interested her until now. Now, something about it caught her attention.

"Hydrogen..." She muttered, looking at the square in the left hand corner. Her eyes travelled across to the right hand corner. "Helium." They moved back to the other side. "Lithium... Beryllium..." Again, her eyes travelled to the right. "Boron, Carbon, Nitrogen, Oxygen, Fluorine, Neon."

As she began to pat her body dry, she tried to work out why this pattern of words struck a memory. She pulled on Leonard's robe, tying it round her front, wondering if it had something to do with him. Something told her it did, and she bit her lip in concentration, but still nothing came to mind, except the vision of him wearing his robe. This made her smile, and she walked slowly to his bedroom, like she had been doing frequently ever since he had left for the North Sea - more frequently as his return grew closer and closer. She was beginning to get excited by some of the little things now that she knew he'd be back soon - and that included seeing him in this robe, which he'd 'forgotten' - though she knew he'd left it for her on purpose.

Closing and locking the door as she entered his room, she looked around, the list of elements still bugging her. With a sigh, she walked to his dresser, stopping to shrug the robe off before opening the middle drawer to pick a t-shirt. As she leant forward slightly to pick one, she giggled as she thought how much Leonard would enjoy this sight.

She picked up a red t-shirt, and sat on the edge of the bed as she unfolded it. And then it hit her, as she saw the periodic table on the front of the t-shirt, and a grin filled her face. That's why she remembered it.

They had been lying in his bed. He was finishing a paper and she was reading the comic book he had finished the night before and left on his nightstand. Once she had finished reading it, she had turned to him, and simply watched him work on his paper. Then, she had begun to trace little patterns on his arm, something she liked to do whenever he wasn't wearing his hoodie and jacket over the top. He'd given her a smile, and a quick kiss, and said he was almost done with the paper. She spent the next half hour just lying on her side, propped up on one elbow, enjoying how serious he looked and how cute it was. He'd closed his laptop, placed it on the nightstand, and then put an arm around her. She told him her thoughts on the comic book, and he explained the things that confused her, or parts that hadn't made sense as she hadn't read the previous issues, while she drew patterns on his chest.

Then, Penny had started to trace around the periodic table on the t-shirt he was wearing.

"There are so many elements." She said, tracing the letters 'Kr'. "They're elements, right?"

"Yeah," He chuckled, stroking her arm.

"Krypton." She muttered, raising a brow. "Isn't that from Superman?"

"It is," Leonard grinned, impressed that she remembered. "But it's also an element. 36 - Kr. Krypton."

"How do you know which number it is?"

"I know all of them." Leonard shrugged.

"Why?" Penny looked up at him.

"I'm a scientist." He planted a kiss on her forehead. "We had to learn the first twenty when I was in school. But my mother said that wasn't good enough, so I learnt them all. Which in retrospect was good, because it makes my work easier."

"You can't know all of them."

"I know, it's sad. It's not as cool as knowing all the lyrics to... I dunno, a Beyoncé song... but yeah, I do know them all."

Penny looked at the printed table on his t-shirt for a few seconds, and then back up at him. "All of them, in the correct order? Atomic number, symbol, name, atomic mass?"

"How do you know about atomic number and atomic mass?" Leonard smirked.

"Shut up." She tapped his nose. "Do you?"

"Yeah." He grinned. "I do."

She raised her eyebrows, and swung one leg over the other side of him as she sat up, straddling him. "Prove it."

"Mm," He placed his hands on her thighs, rubbing them gently. "Right now?"

"Yes," She giggled, hitting his chest lightly. She placed her pointed finger on the first one, Hydrogen. "You don't have to tell me the atomic thingies, just list them. If you get them all right, who knows, maybe I'll reward you."

"Okay. Deal." He grinned at her. "H - Hydrogen. He - Helium. Li - Lithium. Be - Beryllium. B - Boron. C - Carbon. N - Nitrogen. O - Oxygen."

Biting her lip, Penny pulled on the t-shirt in her hands as she remembered how that had ended. That was why she remembered it so well, she thought. She had forgotten how much she loved his amazing, genius mind, and how easily it turned her on hearing him talk about science. Straddling him hadn't been her smartest move, and his hands on her thighs certainly hadn't helped matters. Looking back, she realized he probably knew that him reciting the periodic table and showing off this way would arouse her more than she did by straddling him anyway. So, she had resorted to concentrating fully on the elements he was naming, and following them on his t-shirt intently. Hearing the word 'Lawrencium' had been a wonderful moment.

Getting up from the bed, she opened the top drawer. A few weeks prior to Leonard's departure, she had begun keeping little things like underwear and shorts in this drawer, next to his boxers. She tried to convince herself every day that it was just easier to keep them there, because she didn't want to keep having to go to her apartment just for these things every time she stayed over. But truthfully, she just liked knowing that some of her stuff was in his bedroom. That she kept things where he lived. Smiling to herself, she pulled on a pair of panties and shorts, and looked at herself in the mirror. It wasn't her best outfit, but that didn't matter. She was only going to sit in his bed, watching films on her laptop, before calling him.

No, Penny shook her head. She wasn't going to sit in bed watching films today. She was going to sit in his bed and start to learn as much of the periodic table as she could. He was returning in two weeks, and she wanted to have something to show for the time she spent in his apartment, something to surprise him. Propping herself up with a few pillows, she placed her laptop on her lap, grinning at the idea of Leonard's reaction if she recited the elements whilst wearing nothing but her glasses and his t-shirt. And maybe her boots.