This is just an idea that wouldn't leave me alone.
Disclaimer: Anything that looks familiar isn't mine.

The door to the TARDIS sprang open with a windy flurry of snowflakes and the sound of animated laughter. Martha jogged inside, a large parka wrapped tight around her and brown fur surrounding her face. She was grinning, holding back laughter as she attempted to speak.

"I cannot," she gasped, peeling off her jacket. "Believe that you had a snowball fight…with a penguin!"

"How many times do I have to tell you," the Doctor said over her laughter, entering the TARDIS in nothing but his suit and large black galoshes. He was sopping wet and didn't look too pleased that Martha was giggling uncontrollably at his expense. "It was not a penguin. It was a sixty-eighth century Plutonian Ornithological-"

"It kicked your arse!" Martha gasped through peals of laughter. She was leaning against wall of the TARDIS and slid to the floor as she laughed.

The Doctor frowned at her, sitting down to pull off his boots. "Like you were so much help," he grumbled.

"Oh, come off it," Matha grinned, crawling across the floor to where he sat. The tuft of his hair that usually floated above his face was now plastered to his forehead. The Doctor wasn't grinning.

"You come off it. I know what you're like when you get in these moods!" he said crankily. He pulled off a boot and threw it into a corner.

Martha crossed her legs, her lips still curved upwards. "How d'you mean?"

"I mean," he said, looking up at her with daring in his eyes. "You couldn't keep a straight face if you tried. Blood annoying," he added under his breath.

With faux-insult in her voice, Martha's jaw dropped as she said, "I. Could. So!"

"Couldn't," he replied shortly, hopping to his feet.

"Could so!"

"Nope."

"You challenging me?" She appeared at his shoulder as he began his mad process of twisting and pressing everything he could reach on the main module.

"No laughing or smiling. Starting now." He twirled around dramatically to face her, their noses a mere two inches apart.

"Go."

Martha crossed her arms defiantly, already finding it difficult to keep her face straight.

Okay, Martha. Keep your mind on something serious. Something completely unfunny. Dad's trampy girlfriend... No way, that tan is hilarious. Gran dying... No, they found her on the loo, that's a little funny...

She ran through as many things as she could in her mind, all the memories she could grasp. But it was as though something inside of her wanted to crack her up. Everything she thought of, in some way, was funny. She tried to think of all her adventures with the Doctor, all the danger they'd been in — the Pigmen, the Face of Boe, Lazarus, hurtling towards the sun. But nothing was working.

"What're you thinking of?" he asked her. She felt his breath stroke over her face and she bit her lip.

"Nothing," she replied smartly.

"Impossible," he said. It was odd; it was his usual, expressive tone, but his face was completely blank. "The human mind is constantly shifting, progressing, absorbing, question. Round and round for their entire lives. Like a dog casing its tail." Martha let go an odd strangled snort. "THAT WAS A LAUGH!" the Doctor roared, clapping his hands.

"Was not! For all you know it could have been a sneeze!"

He narrowed his eyes, his face stony. Not a muscle twitched, but he couldn't really keep the laughter out of his eyes.

"Fair."

Martha's treacherous mind began to return to the penguin ordeal. Her nostrils flared as she attempted desperately to keep her self-control. "What are you thinking of?"

And then suddenly, before she could realize what had happened, he was kissing her. His eyes were closed, soft lips were pressed firmly against her own and slightly puckered. Martha's brown eyes were completely circular and crossed, staring at the man now on her face.

"What in the hell was that?" she asked when he pulled away, all sign of the giggles having gone.

"It's what I was thinking about," he said simply, turning back to his TARDIS. "Where'd you want to go now? Feel like a little 19th century French revolution? Or maybe 23rd century Australia… they have a kangaroo as Prime Minister, you know…"

Martha was staring at him as he leapt around the controls, immobile as a steady warmth spread through her body

And slowly, she smiled. The Doctor looked at her when she didn't reply.

"I win," he grinned at her.