Rose Tyler liked chewing gum. And, what was bad about that? It wasn't unhealthy, satisfied hunger, and kept her breath minty fresh. She always kept a pack in her pocket and a stick in her mouth. Sometimes she ran out, only having the single stick in her mouth until she could buy more (which was pretty hard sometimes, considering all her travels with her silly old Doctor) but she lived.
The Doctor liked chewing gum. He didn't set out to buy it, or ask around to see if people had any, but if he saw someone with some he might think to himself, "Hm, I'd like some of that." He wouldn't hesitate to ask.
One day, The Doctor and Rose were in a small-ish Earth café in London. It wasn't a place of great importance, but it was a place each of them liked and they often sat together drinking coffee, perhaps sharing a muffin. Anyway, on that day The Doctor was taking a particular long time with his coffee and Rose was finished, so she popped in some gum, her last piece she was disappointed to see, and chewed energetically as her and The Doctor had one of their conversations about random pieces of nothing, which they both enjoyed. The Doctor finished his coffee and then looked at Rose with narrowed eyes.
"What are you chewing?"
"Gum," said Rose. "What do you think it is, brimstone?" she laughed and grinned at him, and he grinned back.
"Alright, then," said The Doctor, and he placed his hand palm up. "Gimme some."
"None left," said Rose with a careless shrug. "Sorry."
"Give me some of that, then," he said, nodding at her mouth. "You've got some in there."
"Yeah, but Doctor, it's been in my mouth," said Rose with a laugh, raising her eyebrows as if he were mad.
"Yeah, and I don't care! You're not poisonous now gimme!" He was acting like a child begging for sweets, but neither cared. That was their relationship, their funny friendly relationship, even though Rose felt so much more for him and Rose made something burn in his hearts that he had not felt in a long time ... love.
"Alright then, if you want," Rose shrugged, and she took out her chewing gum, pulled in in half, put one half back in and threw the other half in to The Doctor's mouth which he had opened expectantly. They chewed together.
"Good flavor," The Doctor commented. "What is it, mint?"
"Yeah," said Rose with a nod. "Mint and spit ala-Rose-Tyler."
"Must be the spit that gives it the flavor, then," The Doctor joked, with a grin, a laugh and one of his usual boyish joking friendly winks.
"Still cant believe you took something that had been in my mouth," said Rose, though she was smiling and laughing as she shook her head.
"Well as I said, you're not poisonous, far from it," The Doctor said with a shrug, and he put out his hand. "Now, on with our next adventure?"
"On with it!" said Rose, giving him a smile that was so much more than a smile, taking his hand, and leaning against his arm.
Together they walked hand in hand back to the TARDIS to see what the future held for them.