It's those five words that start to crack the ice around his hearts.
He's been expecting the standard words, the words that everyone else has said, those words that make his hearts sing, but at the same time make him crumble, because so many others have said them, and so many people have died because he let them in, let them whisper those words...
It's bigger on the inside.
But this is Clara.
She steps into the TARDIS, her face alight with wonder, her eyes growing large and suddenly so childlike that his hearts ache. She is no more enthralled by his beautiful blue box than his previous companions have been. He waits for the words. The words that ache and soothe him, the words he lives for and dies to.
But instead she grins. "It's smaller on the outside," she whispers.
This pulls him up short. An odd sound, half way between a laugh and a gasp. "That's a first," he manages.
And it is.
Humans, he has learned over the years, are reasonably predictable creatures. They love and laugh and dance. They like routine, and despise change. Their lives are brilliant, burning candle flames, sputtering and flickering in the wind for a brief, beautiful second before vanishing. They are fleeting and sparkling and brilliant.
But Clara is different.
She's not predictable-she is a wildfire, fierce and burning and passionate-she wants to run and find adventure, she wants to dance and run and run and run. She chases after strangers in the night, she protects small children from snow, she teases the, with stories of fish and swimming, she's charming and sweet, and brilliant...
Now you're getting sentimental, he reprimands himself. You can't take her with you, remember? You don't do that anymore.
But she's here, isn't she? She's excited and asking questions and running around the TARDIS as if she owns the place already.
In truth, he knows she ensnared him into the trap of world-saving again the instant they saw each other. She captivated him almost as much as he captivated her, and there was just something so familiar about her voice that drew her to him. Before he knew what he was doing, he had been pulled back into the role of the grandfather: sweet and stubborn and childish. But there is something more to it, something that makes him feel alive for a second. Something that burns within him like a fire.
Clara does what Vastra and Strax and Jenny have been trying to do for so long: wakes him up.
He put on a bow tie. He reaches up and fingers it now. He can hear Amy's voice in his head, laughing.
Bow tie, get rid!
Bow ties are cool!
It doesn't ache as much, somehow. He smiles, a little.
He's been spending years up on this cloud, watching the world from a distance like a Timelord is supposed to. Watching the humans burn and destroy themselves and fight and hate and die. He's been sitting back and letting it all happen, because the universe just doesn't care and someone always gets hurt when he interferes. Someone dies.
But not today.
Not if he can help it.
He looks back at Clara. She's still smiling, in wonder, that flirtious twist of her mouth sending the fire through him again, the gentle curl of her hair bringing out the grandfather. He wants to brush her hair out of her face and make her laugh.
She's so young, but there is something about her that is old. There's something about her that is timeless. Something that enables her to match his wit and questions with ease, something that lets her tease him so much that he wants to scream.
But it delights him, too. He hasn't met someone so clever in a long while.
She had him from the start, if he's honest with himself. But it is those five little words that did it. The words that mean so much and yet are so different then any words that any other person in the universe had spoken.
It's smaller on the outside.
Clara is new and bright and brilliant, a fire that melts the cold ice he has become.
Five words that define him in away, no matter what form there in: bigger on the inside or smaller on the outside, they encompass the scope of his world. The boy and his box, off to see the universe. The old Timelord and his TARDIS.
Which is why he wants to find her. Find her and take her away with him, to see the stars. She's bright and brilliant and she melts the ice and darkness around him with her fire.
"Watch me run!" He shouts to the emptiness of the TARDIS, and laughs until he cries.
Because this girl, this mysterious, beautiful girl made him care again.
And all it took was five little words.