On his way back to the TARDIS, the Doctor scooped up the little round fur ball without breaking stride. "Aren't you just a beautiful thing, with your no head or arms or legs." He cradled it to his chest, petting it absently and enjoying its low, soothing purr. "You want to meet the Ponds, don't you? Of course you do. I think I'll call you a tribble after that little trembly purring sound you make." He unlocked the TARDIS and stepped inside.
The tribble chirruped and shifted in his grasp. "Yes, yes it is bigger on the inside." He held the tribble up to eye level and rolled it around in his hands, examining it thoroughly. "Though I'm not sure how you can tell that, what with no eyeballs and all." The tribble chirruped again. "No, I don't hold that against you. Eyeballs are completely optional."
Depositing the tribble on the console of the TARDIS, the Doctor began twisting dials and shifting levers. The TARDIS made its wheezing sound as it landed. The Doctor rounded the console, appearing from the opposite side. "Oi!" He pointed at from the tribble he had brought aboard to three others that were sitting there purring happily. "There's four of you." He put his hands on his hips. "I didn't invite your friends. And how did they even get on board? No, no! Don't answer that. I'll figure it out. Now, don't go anywhere," he admonished the little fur ball as he bounded to the door to the TARDIS. "I'll be right back."
When he returned, he backed into the TARDIS, talking to Amy and Rory as he went. "They're really the most beautiful things. Well, they're just fur balls, really, but don't say anything about that. I think the first one I brought on board is a bit touchy about not have any appendages." He was so busy talking that he didn't notice the wide eyes look on the Pond's faces nor did he notice that there were no longer merely four tribbles, but hundreds of them. He tripped, landing hard on his bum. "Oi!" The Doctor looked around. "Oh, this is really too much." He looked up at the console and gave the first tribble he had brought on board an admonishing look. "Three friends was fine, even if you didn't ask, but this. No. No." He stood up. "They simply have to go." The Doctor made a shooing gesture. The original tribble chirruped. "What? Oh, that's lovely!" the Doctor exclaimed.
Amy tugged on the Doctor's arm. "What are they saying?"
He turned and looked at her with glee. "They're pregnant!"
"What?" Rory asked with a little shake of his head.
"Apparently these little fellows, oh..." He turned to the tribbles and sketched a bow. "Apologies, ladies. Well, that's not entirely accurate either." The Doctor shrugged. "Anyway, they are born pregnant and when you feed them..." he gestured around, "this happens."
Amy laughed. "Then quit feeding them."
"Right, the thing is," the doctor bent and scooped one of the tribbles up in his hand, "I didn't feed them to begin with." He cocked his head to the side as he listened to a series of burbles and chirrups. "I see! It's the Artron Energy of the TARDIS that they're feeding off of. That explains the rapid reproduction rate." The Doctor bounded over to the TARDIS' console and began flipping levers and punching buttons. "It's time to get you beautiful things back home."
The TARDIS began wheezing, and they were off, back to the home world of the tribbles. The Doctor decided he'd call it Tribblefrey.