When she's concentrating she sticks her tongue out just a little bit, right in the corner of her mouth. It's awfully distracting, especially when he's trying to teach her how to change the light bulb on top of the TARDIS.

"Like that?" she asks, twisting the final screw on the case into place.

"Yes, exactly like that."

She beams at him, proud of herself for a job well done. "Thank you for teaching me."

"Thank you for wanting to learn."

He hops down first, and then takes her hand and helps her down after. He holds her for just a moment too long before planting her feet back on the ground.

"Can you teach me other things about the TARDIS?"

He knocks his elbow into hers accidentally as they walk around the side to head back inside. "Like what?"

"I don't know... everything?"

He laughs at the cheeky little grin she's giving him. "That's a big ask. It'd take a long time."

"Well we've got three hundred years," she says casually, and he stops dead in his tracks.

Three hundred years. Three hundred years. It's still not much to a Time Lord, but from a human perspective three hundred years is an age.

She takes a few extra steps before realising that he's stopped, and when she spins back to face him her hair swishes around her shoulders, falling in soft curls over her collarbone. "What?"

He knows that he shouldn't say this, but he can't help himself. He's been down this road before, so many times, and every time it ends the same - but he just can't help it. "You want to stay with me for three hundred years?"

She takes a step towards him. "I want to stay with you for the rest of my life."

He almost crushes the old light bulb in his hand. "Ariel -"

"Doctor," she says, pouting and mimicking his serious tone.

He shakes his head. "You have a home, you know. You have family, and friends, and -"

"And I'm happier here," she insists, grabbing his hand. "Travelling, with you."

He should tell her to stop. He should tell her that he has to take her home, right now, that's it, time is up and they have to go their separate ways while she's still in one piece. He should tell her that he's not going to entertain the delusions of an emotional sixteen year old who has run away from home with a madman in a box and is just too busy to be homesick yet.

Instead he smiles at her and says, "Okay."

"Okay," she repeats, smile dazzling.

"But we are going to get you home eventually!" he tries to amend, even though he knows it's already far too late.

She's already walking off again, swinging the sonic screwdriver lazily in her hand. "Yeah, sure. But before that, where do you wanna go next?"