*enter generic disclaimer here, you all know the drill*
Regenerative Properties
"S'then what?" Donna Noble asked, cradling her teacup in her hands. She and the Doctor were exchanging stories. Needless to say, Donna felt the Doctor's stories were far more entertaining and impressive than her own. The Doctor was telling her about Bad Wolf, and how he'd had to absorb the time energy from Rose Tyler with a kiss.
It pained him still to talk about Rose—he'd left so much unsaid, undone. Yet Donna loved hearing about the blonde. Whereas Martha would always get jealous, Donna knew these stories were part of the healing process.
The Doctor smiled wistfully. "My body couldn't handle the energy, so I regenerated."
Donna repeated the word questioningly.
"Yup!" The Doctor popped the 'p' sound, grinning like a fool to distract himself from the empty hole in his heart. "When one body's about to die, I form another one. Brand new, just for me. Well … not brand new—I mean, it's my own unique body, but I don't start off looking like a little kid or anything."
Donna smiled. "So you're proud of this regeneration thing?"
The Doctor tilted his head to the side. "Of course! Silly little human-ape! It's superior Time Lord biology. How many animals can you think of that regenerate? I can even grow back limbs, within 24 hours. Or limbs can grow back me!"
"Yeah?" Donna snorted. "Y'know who else can do that? Worms. And starfish. A starfish, that doesn't even have a digestive system. You can cut the head and tail off of a worm and leave the middle third, and it'll regenerate into three worms. And worms aren't limited by any 24 hours. So don't you go on about superior Time Lord biology to me, Mister!" The grinned a triumphant, snarky grin.
The Doctor looked affronted that Donna had insulted his people's "superior" biology, but only for a moment. He grinned at his companion. "Sassy Donna Noble," there was appreciation in his tone. "Just think of how special we are then, an ape and a worm traveling through time and space."