The TARDIS whirred with noise and sound as the Doctor weaved around the console, flicking levers and pulling handles with youthful vigor. Amy watched on vaguely from where she was seated on the stairs, her eyes glazed slightly, as though in thought.

"What's wrong with you, Amelia Pond?"

"What? Oh, nothing." she replied, shaking her head quickly. "Just thinking, that's all."

"Thinking?" the Doctor repeated in mock surprise. "Well, there's a first time for everything, I suppose."

Amy either didn't hear or ignored the jibe. Brow slightly crumpled, she hesitated, then asked:

"Doctor? Why do Time Lords have two hearts?"

"What?" he replied shortly, glancing over at her with bewilderment.

"Why do you have two hearts? I mean, you're close enough to human in most other ways..."

"That's what you were thinking about, was it?" he laughed, turning back to the console. "In all honesty, I don't entirely know."

He frowned slightly, and sat down on the stairs besides Amy, head tilted to the side with a thoughtful air.

"The role of the Time Lords was always to govern and observe the universe, but I suppose you can't appreciate the universe in its entirety without an extra. So many and so much to love, you know? Plus, it's always handy having a spare heart for when the other gets broken. Broken hearts tend to be a bit of a side-effect of time travel. That's my experience, in any case..."

Amy nodded slowly. "Is that what they were like, then? The Time Lords? All like you - throwing their emotions around, and a bit mad?"

"Mad? Oh no, that's just me." The Doctor replied cheerfully.

"What were they like, then?"

His face darkened.

"They're a funny bunch, Time Lords. Flawed and full of darkness, but with such capacity for hope and goodness and wonderful things. And they were smart, too, incredibly smart. Awful dress sense, though." he added in an attempt at comic relief.

"Do you miss them?" Amy asked quietly.

"Every second of every day." he answered immediately.

"Hence the two hearts, huh?" Amy suggested. "More love, more loss?"

"Alternatively, it could also be because the gravity is stronger on Gallifrey, and the second heart is to counteract the higher blood pressure and maintain circulation." the Doctor admitted.

"I prefer the first story."

"Me too, Amelia Pond. Me too."