"Clara," the Doctor whispers. Fingers touch her shoulder, then graze up to brush her hair away from her neck and face. "Clara," he tries again, and gets a vague unhappy mumble in reply. He sighs and sits on her bed, stroking light fingers through her hair and looking down at her with something that is definitely fondness and something else lighting his eyes. He lets her register his presence slowly. Her eyes squint open.
"Doctor?" He gives a slight smile as she peers thickly up at him. "What are you… what time is it?"
"Two oh five in the morning," he supplies, "give or take thirty-three seconds. Thirty-four. Thirty-five." His smile twists into a smirk. Clara struggles to absorb this information, still squinting, a small frown working to find its way onto her sleep-addled face. She rubs at her eyes with a hand.
"You want me to explain why it's two oh five in the morning?"
"Yes - no. Wait, what?" Clara muffles a yawn with the back of her hand, which rather detracts from the stern look she'd been trying to give him.
"Well. The rate at which time progresses from one person's perspective relative to another's depends on each person's velocity relative to the speed of light, so while it might seem to be two oh five - no, six - in the morning for you that does not necessarily mean-"
"Oh my God, Doctor, it's two in the morning."
"-although a dominating view among physicists of your time is that- what? Yes, that's what I'm trying to explain." He frowns at her.
"What…" Clara presses the heel of her palm into her forehead and rubs at the lines gathering there. "No. I don't care. Why are you here, in my flat, at two in the morning?"
"Oh! That's considerably easier." He shoots his cuffs and smiles. "I felt like seeing you."
She stares at him for so long that he starts to fidget. "What?"
"You felt like seeing me…" she says slowly, "…at two in the morning."
"Why does the time factor into it? I always feel like seeing you." He stares at her quizzically.
Her face softens, and Clara relaxes back into her pillow and closes her eyes. The Doctor hesitates. "Clara?"
She sighs, but her mouth is smiling. "Go on, then. Explain relativity to me." He brightens immediately, and she scoots over to make room that he eagerly takes up within seconds. He wiggles around to get comfy and twists his head to look at her; Clara rests her cheek on his shoulder and closes her eyes again. He finds one of her hands with his. Her fingers slide easily between his. The Doctor smiles, then takes a deep breath.
"The simplest explanation is to think of time like one of the three geometric dimensions. For example…"
Clara fell back asleep just as he was getting into the specifics of entropy and the basic schematics of time travel in relation to this. The Doctor found he didn't mind in the slightest.